Flooring Tile - Grout on fiberglass shower basin

I bought my home about a year ago, and the previous owners had renovated the bathroom and installed a new shower basin and tile (I think they hired a contractor on the cheap). The tile basin is fiberglass and is textured. The problem is, they left grout haze behind and it's in the texturing and some of is a pretty good layer. I've tried regular cleaners, barkeeper's friend, CLR, and now a regular grout haze remover and none of them are making a dent in this stuff. Plus, I'm getting concerned that the abrasive stuff is removing the gelcoat on the fiberglass. I can't even get it off the smooth parts of the shower, though. Any suggestions?

What stinks is that they didn't leave any tiles or grout behind, so it's not even like I can replace the basin without redoing the whole thing.

Any suggestions would be much appreciated at this point!

Sulfamic acid crystals might work? But test first.

It's recommended to have spares for possible repairs, but you can't just remove the pan and install tiles anyway. A bit more to it than that. If the floor needs to be removed, rip it all out and start over.

Jaz