The only Polaroid film that was sharp for medium format was type 55, and you needed to check the negative with a loupe. Polaroid was mostly good for checking lighting and getting client sign-off before committing to film.
I've mostly use type 100, but if you think that is not sharp, you'll be in for a surprise with Instax (it's similar to SX-70 film).
I don't think you can get a client sign off with Instax, it will not look like the final product.