The problem isn't a diaphragm clutch, been running them for years with zero issues in place of the overly stiff OEM setup. I got three daughters, teaching them to drive stick was much easier lol.

Odds are it's the flywheel as others have mentioned. We already know a "mirror finish" is the wrong one for a flywheel. Now what damage that may have caused, to the disc, well you will have to inspect and make that call.

With this setup I would assume nothing was done right before you got it so go from that position and fix what we know is wrong, then test.