I’m only responding in the spirit of the wording of the above question, which is that, yes, there are a few issues, but they are simply Plasma bugs — and mostly 15-minute bugs, I would say. Other than that, everything went smoothly, but then again, I update Manjaro the smart way, and I pay attention to all Announcements threads — including for the Testing and Unstable branches. 
Everything depends on whether these are qt5 or qt6 applications. kvantum was updated to qt6, so if you want continued kvantum support in qt5 applications, then you need to install kvantum-qt5 — it’s an optional dependency of the qt6 version of kvantum now.
Negative. In Plasma 5; those were handled by khotkeys, which was dropped for Plasma 6.
Select — or install, if you don’t have it on your system already — one of the Nerd-flavored fonts, such as Noto Sans Mono Nerd.