There's similar ideas to be able to write over an existing annotation so you can selectively erase.

This suggestion is different

I want to be able to flatten an annotation so it's not enormous! I like to do a detailed drawing, and then I'm done! I don't need my exported PDF to be massive, and I'm not going to come back and edit.

By not flattening, the notebook gets quite sluggish!

Use case:
I use GoodNotes as a laboratory notebook