Wow! Can’t wait to see what you come up with. I know there are some tiny sinks out there, I’ve watched a lot of tiny home builders’ shows and they always find a way to make some tiny bathroom accommodations.
Wow, that is an ambitious project! Looks like you will get it done in a few days. You can also check the floor for any water damage very easily but it looks good. Progress and after pictures are a must on this since it seems there is a lot of struggles with the bathroom for many owners especially with a rear bathroom.
Wish I had a little tub though the shower is so large I don't think I could replace the pan unless I have the glass door & walls cut higher.
I wouldn’t go thru all the pains you’ve gone thru without custom building a replacement to your exact liking. If you can’t do it yourself, Id suggest going to a custom cabinet shop and getting it made to your exact ideas.