Sorry, the closest i came up aren't good solution but may help in your search.
Vanilla OS 2 (based on Debian) but it is under Gnome DE and in beta phase. Very begginer friendly. Maybe once it go out from beta it will supports other DE ? So check it around 6th month later or 1 year ?
But the problem is that their community is very small. If you want something stable, it's better to look for bigger community so you can benefit from their support and user's problems
There is fedora kinoite but you don't want anything related to IBM. That was the best compromise i can found.
NixOS but i don't know it. I'm affraid it will be a DIY distro at the beggining with the config file. But it will probably meet all your criterias.
Or the same OS from my steamdeck :
Steam OS ? It's an immutable OS based on Arch and support KDE by default. Full support of flatpaks. Only downside, i dunno if it supports other machines than the steamdeck. Nor if it uses the latest linux kernel. Maybe some variants ?