Might be too difficult to gauge and implement, but there should be a limited number of long rests you can take before the bug in your head takes over. Maybe warning signs along the way and/or incremental changes to the characters (or Con checks to resist these small changes).
That's not going to work, because it would be too easy to get into a "Walking Dead" state where you reach "Number of Long Rests Before Transformation -1", but are unable to complete the process of removing the tadpole without needing to take another long rest, and so your campaign will be over, forcing the player to start a completely new game.
If you don't think that will happen, look at how many people, in early access, are complaining about losing progress because they are relying ONLY on autosaves and never saving manually.
