show the progress in WAILA because (especially as a newbie) it is hard to determine when the leave is finished

Honestly, this is more of a design problem than a WAILA problem. I should just make the final stage look different enough from all the other stages so that you notice without needing that additional information from WAILA.

Yep this would be a good alternative! Maybe add some "berries" in the leaves texture (like with the oreberries mod) when it is finished.

As a side note: I love your mod, it's really fun!

May be a little bit of an effort (= balanced) but maybe have gold leaf particles drop occasionally from the bottom of the leaves?

wow, you're fast!