I've yet to see a serious comparison of grammarly alternatives.

I feel there is already a lot of potential for using ChatGPT to replace most written language tools. For some actions, like rephrasing, I find that some prompt engeneering is necessary to precisely modify my "text" and/or get a useful "commentary" on it. As you said, iteration is also key. Just make sure to use GPT-4 (not 3.5).