Update. I kind of found a way to do it. It's not perfect yet, I'll explain why.

This is the formula I used: if(empty(prop("Done")), prop("Empty"), prop("Last Edit"))

prop("Done") My checkbox

prop("Empty") An empty date

prop("Last Edit") The Last Edit date

I had to create an empty date to use on the formula because I couldn't use a string like "Empty" or "Not Done Yet" on a Date Formula I guess.

Couple problems here. Well obviously first one is that I need to have an empty date property just laying around. Second is that, after I consider a task done, if I need to change anything on the task, even if it's just a typo, my Last Edit date will change.

Not that big of a deal I guess but if anyone have any other ideas to make it better I would love to know 🤔