I think it's a bit much saying ''It seems a cure is impossible'' after just trying to get rid of it with the help of a few characters under lvl 5 and a rogue mind flayer xD
Correct. Normally removing tadpoles in D&D isn't an issue if you get to them on time...even greater restoration(a relatively common spell among healers) can do the trick. Not in BG3 it's a bit trickier because there is the stasis holding them back and that also interferes with normal removal methods but it's still a huge leap to say it's impossible. Literally nothing is impossible in D&D. There are always some gods you can bargain with for some miracles should you need them.