kcmshell5 fonts is not ignored here with a fresh 15.1 installation, though started via Konsole there are three libGL/nouveau errors. When you wrote “kcmshell5 fonts is simply ignored” how were you starting it, Konsole, Xterm, Alt-F2, “Configure Desktop”, something else? Try verification:
I have 14 inch, 2560x1440 native resolution
with 15.0 I had a super small login screen,
I tried different solutions, but especially with a second monitor attached that is not hdpi, nothing worked useful
so the best was to accept the small login screen and then have a low res desktop resolution that works with the external 20 inch 1920x1200 monitor
the new KDE does some magic now, the login screen is normal sized,
I need now to use the native resolution but I have to set DPI explicit in the control center, otherwise everything is messed up
need still to test with an external monitor, and how applications look that simply ignore the DPI, (those exists, Davinci Resolve for example, a 3rd party qt application)