Lot's of great answers already. I'll just had my thoughts and hope it helps.
Choosing a Linux distro is fucking hard, but the good thing is that you already have a DE (Desktop Environment) that you like, KDE Plasma (KDE is the community, Plasma is the name of the DE) and it's my favorite.

1 - Manjaro was my first distro for daily use. I would not recommend it, i don't think it's stable enough to get into linux. Would not recommend any Arch based distro.
2 - OpenSuse is an old distribution, but not beginner friendly, so maybe not a good idea to dip your toes into it.
3 - Fedora is well established with lots of documentation, a big community and a 6 month update release model that should give you the newest features very fast while still maintaining stability. I don't recommend the Atomic distros. If you've already installed and it works then stick with it for a while.

There are also the Ubuntu based distros like Kubuntu, KDE Neon or Tuxedo OS. Ubuntu has probably the largest user base, so documentation is abound everywhere regardless of the distro you pick.

You're already testing out different distros, try to daily drive for a month and read up on what makes them different. In general it's how to install software, the release model ("Long Term Support" or "Rolling Release") and the core system. Apps are installed on top of the system and right now come in a variety of formats. I strongly recommend that you enable Flatpak on the distro you chose and use the Discovery app for software management.

Edit: Added "Tuxedo OS"