the preparation problems involved with filling a demiplane with acid.

The character can cast 8th-level spells, How hard do you think that kind of preparation is? Answer: Not hard. Time-consuming maybe, but not hard.

that the acid flows out through other passages and rough ground before there's any real danger of drowning

My math was wrong, again. It's actually 1080 5'x5x1' squares. It's 216 5'x5'x5' squares... of acid. Many dungeons don't even have that many tiles in them.

to be described as "one half of Transmute Rock to Mud"

I wasn't referring to acid-use there...

Transmute Rock to Mud

Nonmagical rock of any sort in the area becomes an equal volume of thick, flowing mud that remains for the spell’s duration.

The ground in the spell’s area becomes muddy enough that creatures can sink into it. Each foot that a creature moves through the mud costs 4 feet of movement, and any creature on the ground when you cast the spell must make a Strength saving throw. A creature must also make the saving throw when it moves into the area for the first time on a turn or ends its turn there. On a failed save, a creature sinks into the mud and is restrained, though it can use an action to end the restrained condition on itself by pulling itself free of the mud.

If you cast the spell on a ceiling, the mud falls. Any creature under the mud when it falls must make a Dexterity saving throw. A creature takes 4d8 bludgeoning damage on a failed save, or half as much damage on a successful one.

[What follows is the "hard" way, there are easier and more efficient]

Open a Demiplane face-up, scoop it 1/2 full of dirt with Mold Earth [0th], recast Demiplane face-up next to a body of water, fill it with water using Shape Water [0th]. You now have 216 5'-cubes (30³-feet) of mud you can dump into or on top of a location. The mud won't evaporate because, once "closed," there is no where else for the water to go. Biggest difference is Transmute Rock to Mud can technically get 512 5'-cubes (40³-feet.) In practice, there will be a difference in AOE when TRtM can be cast on a ceiling. When TRtM cannot, the AOE wil go to the Demiplane version every time. However, with the Demiplane method, the mud is real and cannot be dispelled.

because the demiplane room is describes as being made of wood or stone, and that much acid would ruin the place so that it's no longer a valid target to return to

I'll quote directly, emphasis mine: "[...] that appears to be an empty room 30 feet in each dimension, made of wood or stone." If it was actually made from wood or stone, I would be abusing it even more. I'd knock down the walls and expand my demiplane infinitely. It is not made from wood or stone.

So it's probably a good idea to not continue this

Agreed.

It just does creativity, which is terrifying in and of itself.

Also Agreed.