For KDE development I would recommend the KDE Neon developer edition as it has all the tools you need to get started.
KDE
KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
Plasma 6 Bugs
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
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?
I just installed Neon on my PC a couple months ago and it's my daily driver. Yep, it's still a thing. 😁
I'm on Arch and very happy with the experience
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.