Following along with Paul's method, I took a day or two away from this project to give my subconscious time to be creative.
In my mind, detailing the dungeon environment and detailing the monster design go together; you have to know who/what is using the space to figure out what the space should be. As it stands, the mound should be built primarily to the logic of the termites, perhaps with some influence from the hanging moss thanks to its possession of the termite queen and king. The faerie and sundew are strictly external invaders, so while they might have repurposed some of the areas they control, those should still be evaluated from the termites' perspective first.
As an aside, moments like this are my counterpoint when people say that low fantasy is boring. R.E. Howard's Conan stories are low fantasy, yet Conan himself was clashing with dastardly sorcerers or trying to escape the wrath of some eldritch abomination in pretty much all of them. A low fantasy setting doesn't proscribe wondrous adventures, it serves as a point of contrast to make such things more wondrous, and so I, a strong proponent of low fantasy and limited magic, now find myself trying to get into the mindset of a giant bug horde.
A read through the wikipedia page on termites raised the following interesting details:
the walls of termite nests are generally built from a combination of soil, minerals, and fecal matter known as carton
as mentioned in Part 2, some termites create fungal farms as a source of additional nutrition
the termite hive mind is willing to sacrifice any number of its individual members in defense of the king and queen
the termite king and queen are mated for life, though secondary/tertiary/etc. kings and queens can exist in the same nest, generally serving as workers unless a death triggers them to take over the primary role
in defense of their nests, termites can commit suicide by blowing up their thorax to spread noxious chemicals and/or seal a choke point with their corpses
soldier termites have larger heads evolved for combat, possibly with tubed horns that can spray noxious chemicals or with mandibles that are so purposed for fighting that they can't even feed themselves anymore
Looking at the nutrition information for mushrooms on wikipedia showed that they're an appreciable source of certain metal minerals. It so happens that I was also looking through my Moldvay Basic D&D book recently, which has some cute art on page B41 that I see as a pixie (or more generically, a faerie) teasing a rust monster. Termites are notorious in the real world for their potential to weaken wooden structures, but in order to make them more interesting as a dungeon obstacle, maybe these termites can rip off of rust monsters and eat some kind of metal? Only the workers might be a direct threat to destroy PC equipment, since they go out to collect the metal and then feed it to the rest of the nest's inhabitants. Seeing a huge mound made from a chaotic blend of metal scraps and pulverized stone towering over a ruined settlement sounds like an evocative first impression. Whatever they poop out to use for building the nest would presumably be tougher than actual termite carton, too, so it'd present more of an obstacle to tunnel through it. I'm liking this train of thought, although on reflection, I think I'd like them to focus on eating softer metals; aluminum, brass, bronze, copper, gold, lead, tin, silver, and zinc; and rely on their fungal farming for harder metals like iron or platinum.
Getting back on topic, it seems like there isn't much in the way of specific organization to a termite nest other than what I'd mentioned in Part 2: many choke points to be held by soldiers, the queen cell in the most defended location, and fungal gardens that the termites fertilize with inedible organic matter and feces. At least that leaves me free to be somewhat random in what most of the rooms contain. Since the large heads of the soldiers are meant to block off choke points, I'd imagine they don't have much mobility within deeper parts of the nest, so they wouldn't work as random encounters; I'll rely mostly on workers and vermin for that.
Speaking of which, unfortunately, termites aren't known for forming symbiotic relationships with anything other than their fungal farms. Thanks to the latter, I can see them making some use of shriekers, but aggressive fungi will take a little customizing to be viable as more than an infestation that took over a localized area. Taking a page from the Deepnest area of Hollow Knight, though, I would certainly be open to the nest having copious amounts of regular termites living it in, constantly skittering and scrabbling about in search of wood (or other plant matter) within the mound, perhaps along with regular ants preying on them. They'd be there mostly to unnerve the players and maybe to disturb resting, but perhaps the termites could assemble into a swarm or two to aid the giant soldiers in case of combat.
The faerie's areas would be clear of them, of course, thanks to the monstrous sundew, and I wouldn't put it past the arachnoid fey to have released some swarms of spiders or scorpions, too. Maybe I can have him work with centipedes as well (another non-insectoid arthropod that preys on termites), though on second thought, that might be diluting his focus too much. I'd probably stay away from giant versions of any of those, firstly to avoid devolving the whole dungeon into a giant zoo, secondly because seeing a bunch of normal spiders snap into suddenly formation and attack through the faerie's influence strikes me as a more powerful moment than just having a giant spider pounce.
On the topic of the faerie, here are some wikipedia insights about scorpions and narwhal tusks:
natural predators of termites
excellent vision in low light, possibly among the best in nature
can have compounds in their carapace that glow in UV light
superficially romantic mating habits (dancing, kissing), maybe a little bit of a date-rape vibe with males trying to sedate females while kissing, but surprisingly no sexual cannibalism
historical use as a symbol both of evil and of warding off evil
actually a prolonged canine from the lower jaw with a helix spiral
hollow and fairly light for its size (about 3 lbs./foot)
contains many nerve endings allowing it to act as a sensitive sensory organ
believed to heal, neutralize poison, and cure depression
Putting it all together, getting scorpion legs for the faerie was a wonderful bit of serendipity, and the romantic mating habits with a bit of a creepy side is a perfect fit for a fey creature (and it's a nice excuse to use the tarantella's dancing effect). I'm feeling like the sundew's magical light should be a blacklight now, giving it a more alien feel, and the faerie should act in a somewhat sinister but not explicitly threatening way, probably without giving any specifics about why exactly he wants the hive caller. His tusk prevents him from being affected by the noxious chemicals of the termites (or any other poisons), his tusk and sensitive eyes combined prevent him from being surprised or fooled by invisibility, and he can use his tusk to heal others at a touch (a nice reversal from what's expected of a scorpion's sting). If removed and powdered, it can be used making potions for either healing and divination purposes. Maybe it can even revive the dead, but only if he uses it to do so, to add a drawback to killing him for the tusk.
Now, given the hive mind's devotion to the queen and king, I don't think the hanging moss's influence needs to extend to all of the giant termites. Perhaps some infected workers can be a possible random encounter on the ground and/or first lower levels, and only the lowest level and room 14 are fully infected by it. Again trying to imagine the impact it'd have in play, I like the idea of the players getting used to encounters with normal bestial giant termites before running into an infected one, maybe a worker who stops at the edge of their light with an odd, twitchy posture and clatters its mandibles to draw their attention; if they get close enough, it spasms to release a cloud of spores, which allows telepathic communication between the PCs and the hanging moss if inhaled.
All told, I've got enough of an idea of what's in there now to feel confident making choices about what does or doesn't fit, and I've got some mental images to guide the atmosphere that I want. Thus, let's get to assigning some room content. For this first pass, I'll lay out initial thoughts for any rooms that strike me as having immediate purposes.
1: Some soldiers wait here to seal the exits against intruders.
8: A soldier wait here to seal the exit against intruders.
8a: Small fungal garden. Toxic to mammals.
9: Storage room for workers scavenging to the east. Can be searched for precious metal scraps.
11: Former trophy room. Has artistic treasure (maybe some statuary?) and possibly a potion or two.
13: Major metal hoard set aside for workers to grab while evacuating the queen and king.
14: Huge fungal farm, with countless varieties. Tended by infected workers who don't set off the shriekers. Certain rare fungi can be harvested and sold to collectors for a handsome reward. Need a table of multiple good and bad results for consuming them.
15: Grave of a soldier whose head seals the entrance. Disturbing it releases trapped toxic gas.
16: Special room imbued with magic from before termites' arrival.
17: A constant downdraft carries a sweet smell from above.
19: Fungal garden. Can be searched for precious metal scraps but doing so releases dangerous spores.
21: Soldiers here can move to seal either exit.
25: Checking the nest near the entrance finds marks from climbing gear. Fungal trap room, but there's also a dead adventurer with some valuable items (scrolls? spell book?).
26: The sundew waits here, ready to attack intruders and capable of riding the air down to 17 in pursuit.
28a: War room of the faerie. Large, ornate wooden table (too big to fit through tunnel to 28); chopping it up reduces the total treasure value.
29: Bedroom of the faerie. Weird consequences for sleeping in his bed, weirder ones for sleeping with him.
34: Small fungal garden. Searching yields only iron scraps, but beneficial effects if the fungi are eaten.
35: Choke point from 32 is blocked by termite corpses.
37: Infected soldiers guard the way down.
40: Stash of inedible metals, including platinum.
41: Several infected soldiers guard the way to 42. They don't pursue.
43: Pool in the southern portion of the room.
44: The queen cell, with the queen, king, and several infected workers.
44a: Larder of the termite queen, full of lead and tin. The hive caller sits dormant beneath the hoard.
Phew, I ended up having particular ideas for more rooms than I'd expected at first glance. I suppose that's part of the benefit of having taken a break instead of trying to power through the whole design as quickly as I could, but it does mean I didn't have time today to develop those descriptions.
Next time, I'll start fleshing out the details of those rooms. That'll involve putting some mechanics against the trap/trick/special rooms, so I think it'd also be a good opportunity to put together stat lines for the inhabitants and fill out random encounter lists.