A bunch of them: Kate, Konsole, Dolphin, KCalc, Kdenlive, Okular, Gwenview, Ark, Spectacle, KDEconnect, Elisa and probably a couple more I missed.

Prime works well anywhere tho. I guess you are asking for a GUI to switch between them and/or disable it on a per software basis? For me games via Steam “just work”, they use the Nvidia GPU by default and Lutris has a little switch to enable it. It’s only getting more complicated if you want your Nvidia GPU to fully turn off when not in use.

For my part, I will simply wait until Plasma 6 arrives in the official package sources.

Same. And then I wait a day or two to see if there are any major issues. I do the same for Nvidia updates. I mean I could also just roll back if anything happens, but why bother?