Oh, I am already on Plasma 6.7, it updated to that maybe around a week ago or so! That long press is at least more streamlined that the Compose key, so that’s something! Good to know, thank you a bunch, I didn’t know about that feature :)

I have the original Halo75 (not the V2) which sadly doesn’t support QMK/VIA yet. I remember there was some dedicated app to configure the keyboard (NuPhy Console maybe?) but I don’t know if it’s worth trying to get that to work on Linux and that seems to be basically abandoned/obsolete

Thank you for your answer!

1. I had already tried that before but forgot to mention it, in Mac mode it flips the function/media key behavior as expected; in Windows mode, it does nothing (the function keys still always act like media keys, no matter what)
2. Not quite sure what that would do, maybe I’m misunderstanding you, that seems to just type the angle brackets that are on the layout in Windows mode and nothing in Mac mode
3. I just tried that, and while it does work, it is very unintuitive to me (instead of just Alt+Q to get an ä, it’s now <Compose>, followed by Shift+’ to get a double quote, and then a/o/u to get the umlaut. That would be suboptimal for me at best, but I’ll keep it in mind as a backup solution

I’ll keep looking for and trying some more options :)