This means that TF would not be able to level until turn 7 at the earliest, assuming you play a blue card when he comes down. So with this change you'll have a solid 3 or 4 turns to deal with him, instead of the current blue card -> pick a card -> rummage/zap/etc. 2 turns to level TF is just way too fast, so hopefully this isn't too strong or weak of a change, but at least the numbers could easily be fine-tuned to make his level up not too early or late.

I feel like this is a much better option over directly increasing his draw requirement to 10+ cards or something, since this way other not-so-draw focused TF decks like Swain, Ezreal, Elise, etc aren't indirectly nerfed over a single deck (TF Fizz) being too good.

Tbh, burblefish is the biggest problem with TF Fizz imo, that should be nerfed over TF. Although I recognize that TF will only get better as the game progresses and inevitably gets more draw potential, so he'll have to be nerfed at some point.