After playing some more, expansion potions are absolutely useless against any opponent that has any kind of fast cycle gimmick i.e. lots of accelerants, and it's also terrible against any comp that has compression potions. both of these let the opponent either empty or shrink their cauldron faster than you can expand it, and I say this having managed to get a run with a level 5 expansion potion in deck and a pair of level 5 accelerants, you can't cycle fast enough to outpace any kind of enemy that has the above. I'm not sure how you would solve this without making expansion stronger vs enemies that have slow cycle/cast rates at the very least it would probably be nice to have the expansion scale more smoothly at higher sizes since it's very hard to maintain expansion at a threatening size; once you get really big the potions shrink by enough per turn that the extra space is enough for the new potion thrown in
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Also will second the statement many others have made that the hypnotic swirls are absolutely overpowered, as they basically combine two powerful effects of permanent spacefilling (will eventually brick you due to no more space in the cauldron) and a constant allure drain with 0 counterplay. This makes it just a vastly better slime potion that kinda also has the effect of the allure potion. imo the hypnotic swirls need to either be not permanent or hypno potion needs to be replaced with a hypno item similar to the cowbell nerf.
another suggestion would be to allow the player to choose which 3 "special" potions they have instead of automatically giving them expansion potions, as this would make it a lot easier to try out different builds
This is a pretty fun game its just that it gets kinda frustrating to play when it's both hard to get a build going and most builds will get stonewalled by a handful of enemy comps anyway so why bother.
anyway compensatory bug report for complaints: not sure if this is intentional, but if you use the drain potion to get a high level potion in hand, and then end the battle, you can then use the dupe feature to get a copy of that potion at that level instead of it reverting to it's original level. IMO this is worth just making a feature because it gives the drain potion an extra use and helps to assemble builds for some of the potions that kinda suck and need high level versions to be effective (like the expansion potion).