For well over a decade, maybe 15yrs I've been using Ubuntu and now Pop-OS
I am really interested in starting to getting involved in KDE development somehow as retired software engineer.
Also keen to switch to Arch....
I'm out of the loop, but I used to use Slackware -- largely because the distro didn't get mad when I just installed things from source directly onto the filesystem. No dependency tracking ;)
But I had a lot of experience before that with other distros.
Good choices are probable OpenSuse (tumbleweed), Arch, or Neon. Actually, I don't know the current state of Neon... is that still a thing?
Any distro that allows you to patch and rebuild KDE packages quickly will be good. Personally, I really like Arch for this purpose because working off of PKGBUILDS just rock.