Here's another map of mine, this one hand-drawn, though in my hybrid vector bevel style. It's a swamp, if not obvious, the tree trunks with the black shapes in the center is my attempt at hiding the upper branches and foilage of these cypress trees (you can see the knees around the roots) so you can actually use the trunks for blocking line of sight, but still being able to see what is under the foilage. There's even a green snake in the map, if you can find it!
are you explicitly looking for monsters? because why not use classes who lean into the snake imagary, druids that specialise in wildshaping into snakes, a rogue-assassin-whatever who weilds poisoned or paralysing whips, a ranger with a giant snake animal companion, a wizard who uses summoning to create wave upon wave of serpents or uses fear and hypnotic illusory effects?
Edit: what about a flooded room with sea-serpents?
Maybe have an apple tree or some other "forbidden fruit" in the dungeon. Use it for a theme of one wing.
Maybe a portion devoted to Triglaz (?) the snake god that was all about disease or something.
For a more lighthearted take, maybe a portion that has allusions to famous movies with snakes: Indiana Jones (you could have the pit), Snakes on a Plane (maybe a script above a door with one of its famous lines), Anaconda (a small boat that has to cross an underground lake), etc.
Another option entirely different would be to have a medusa (or The Medusa) as the final boss battle. In each wing, you fight a specific style of serpent. These represent the snakes on her head that imbue her with power. Hence, if you don't clear out the wings, she will have something similar to a Beholder's power - ray of death from one snake, cloudkill from another, etc. Of course, the catch is each snake rejuvenates, so there is a time limit on them clearing halls and getting to Medusa.
I've settled on a suite of seven susurrant serpents for the sake of sibilance.
1. A stalwart guardian naga that is tasked with defending the portal entrance of the place. The PCs have to go through the dungeon to get where they want, but finding the exit will be a challenge. This naga doesn't wish them ill, but he is bound to stop any from passing, so he will fight them. And even if they destroy him, he'll respawn every few days, which makes coming and going a bit more tedious if they need to withdraw to resupply.
Beyond the entrance is a sort of walled desert citadel that has fallen into disrepair. It's surrounded by a wasteland, but the citadel itself was once a waystation between planes. Somewhere in the ruin is a portal out, but finding it will be a challenge.
There will be random encounters with weak fiendish serpents and salt-encrusted spirits, areas where the ground has collapsed into a brackish reservoir, and various buildings whose entrances are hidden behind mounds of sand that take time to excavate. The adventurers are 11th level, so the 'mini-boss' monsters will probably need some buffing.
2. A cruel spirit naga lurks in the ruins, slithering stealthily, hissing mysteriously, and whispering dissonantly. Its spell list lets it sneak up on the party while they're resting and ambush them, then slip away. It respawns if killed, which is good, because in its hidden lair it has half of a tablet that recounts the riddle you must answer to find the exit. If they kill it prematurely, they can always try again later, or just brute-force the search.
3. A tower with numerous balconies and exposed stairways rises high in the center of the citadel, disappearing into a sky that's perpetually night. A mummified couatl circles in the darkness, and swarms of small vipers slither through the tower's walls. A few mummified 'survivors' of when the citadel fell can be found at the top, and they have the other half of the tablet.
4. In a ground-level vault, a hypnotizing cobra guards treasures in a chamber full of traps. Its gaze can force you to move into pits and spikes and such, and if you resist the gaze, it spits venom to try to blind you.
5. A huge icy constrictor swims through the brackish reservoir, and as the PCs cross makeshift bridges, it can shake bridges by smashing into them, tripping everyone into the water, or it can lunge, bite, and drag down, or shoot spikes of ice upward from its back.
6. A poisonous hydra protects the spirit naga's lair.
7. The medusa-marillith whose gaze turns things to salt guards the exit portal. You can attempt a stand-up fight, but if you can answer the riddle, she will let you pass.