This is more of a public note to self, but if anyone else had screwed up fonts, default cursors, and missing minimise/maximise buttons in flatpaks on KDE Wayland, put this in your /usr/share/xdg-desktop-portal/kde-portals.conf:

[preferred]
default=kde;gtk;
org.freedesktop.impl.portal.Settings=kde;gtk;

Then restart xdg-desktop-portal.

Source: https://bugs.kde.org/show_bug.cgi?id=474746#c12

Apparently this will be fixed in 5.27.9 releasing on the 24th anyway, but I've tried so many different 'solutions' and this had been annoying me for weeks.