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 ?

Omg you just described slackware. Join us!

An immutable distro with a heavily customized KDE desktop is Nitrux. Check it out at nxos.org

Psst... Try nixos 😹

debian (mx-linux has a kde version if you want less hasle then pure debian) or opensuse leap on the "stable" side, opensuse tumbleweed if you want more recent packages (i've never had it destroy itself like arch, its been very stable for a rolling distro)

I would recommend void, alpine (kde plasma auto installer may still be broken for some users, works for me tho, also musl so if you need appimages or some very specific applications don't use it.), alpaquita (much stable alpine with glibc if you need appimages), slackware (current only, it is stable rolling, and their point release features very old kernel and packages so I wouldn't recommend it, paldo (stable rolling, gnome by default but plasma installable.), gentoo (if you have time to compile, why not it as stable as rolling can get without it being openSUSE), openSUSE (easiest rpm based (Oracle fork) but still IBM code nonetheless)

Gentoo.

It's rolling release, has stable and testing packages, and users can choose between them per-package (or globally) and it runs or is easily made to run on pretty much everything.

Solus. Snaps optional.

Same recommendation as usual from me :) pepparmint OS , Debian base extra on top

• Siduction
• openSUSE

Slackware current.

Seems that Slackware is what you are looking for.