What are some snake-themed monsters? Constrictors and vipers, obviously. Naga, yuan-ti, medusa, couatl? What are some others? How would you fit them all into a dungeon while making sure each encounter has a distinct hook to set it apart?
Only the DM can choose the hook. We don't know if this should fit into a larger story, or be a little side quest. If it's separate, the easiest hook is the promise of loot: some snake-related item (Staff of the Python)?
If you have a need for a snake themed dungeon, I would definitely consider to homebrew a few snakes. Just take the snakes of the Monster Manual, and change them: give them ranged weapons (breath weapons) or make them larger/smaller. Maybe some snakes can swim, and you add some water to the dungeon. But keep them as regular (if somewhat magical) snakes. And one special BBEG, which could be the Medusa, Naga or whatever.
Also, snakes can squeeze through tiny gaps, so make sure that the snakes have all kinds of shortcuts through tiny holes, while the PCs must take the long way around. To make it harder, put a little curve in the holes, so the PCs cannot see through it to the other side.
Some sort of homebrew giant serpent, serpent swarms, possibly cockatrices. Or even (stretching things a bit) a chimera or dragon. Not entirely sure what you mean by your last question? You can vary encounters based on what purpose they serve in the dungeon. Maybe the Yuan-ti keep some serpents around in an area to help in snake-worshipping rituals. One monster might be a pet. Others patrol the place. A snake-swarm or group of vipers might be part of a (pit) trap. Another might just have wandered into an abandoned section and remains there, tolerated by the intelligent enemies because it is vaguely serpent-like and thus holy. Encounter settings might include pillared-halls, ornamental jungle-gardens (possibly to provide sustenance for a walking/bleeding food-source), magical sanctums, torture-chambers, barracks, temple halls, ruined areas with cracks or deliberately constructed tunnels that are suitable for a snake-creature but too narrow for larger/rounder creatures, fortifications, bathing pools.
You can make environments that cause tactically relevant effects - pool, rivers, rooms divided by grates that ranged attackers can fire or cast spells through, narrow tunnels, areas with lots of alcoves / pillars / statues for stealthy creatures to hide behind (very effective with medusae), elevated areas, regions with magical effects - for instance maybe an area-wide curse that imposes disadvantage on saves vs poison. Maybe an alchemy lab in one section with amphorae of experimental poisons that enemies can use against PCs to cause diverse effects or ailments. Maybe an indoor moat in one place filled with poisonous or unbreathable gas. Intelligent enemies can construct castle-style fortifications with portculli, murder holes, and/or indoor partial-height walls to take cover behind.
Or what if you have a green dragon, and nothing is what it seems? Dungeon full of snakes, in a misty and damp jungle, with the promise of great loot, but actually it's a trap by a green dragon who wants to loot (and eat) the adventurers?
Or maybe Ye Olde Mad Mage lives in the dungeon, and all snakes are actually animated ropes?
I have a small snake temple map interior area, which I probably created 10 or more years ago. I created a 3D mamba head and stuck it in back wall of the temple with an offering pool in front of it. You have to walk or jump across that water-filled ditch, that I presume is full of snakes. I see this a temple ruin in the jungle somewhere. It might help your adventure...
There is a surprise hydra attack in Tomb of Annihilation in an underground river in the yuan ti temple. I've never seen PCs flee so fast from a fight in all my life. They didn't even try to hit the damn thing; it was full blown retreat mode. They cast spells to desperately blow their canoe faster away from the hydra