I would try this thread on the EndeavourOS forum. I imagine that resetting your plasma config is mostly going through ~/.config and cleaning out anything you don't need.
Is there any particular reason you're sticking with X11? I get the impression that there are less issues with wayland on Plasma 6.
Yea, it really is very good. I'm not sure what you mean by gamut tools, but there are out of gamut warnings, gamut masks, histograms, etc.