Ubuntu is bad. Go with any other distro. I generally recommend Fedora for newcomers. Specifically Fedora KDE Plasma Desktop edition: https://fedoraproject.org/en/kde/
If you fear you might break the system and don't have confidence in fixing it yourself, go with Fedora Kinoite. It's an immutable distro, so you can't break the system as easily as mutable ones: https://fedoraproject.org/en/atomic-desktops/kinoite/
While I don't necessarily like Flatpaks, you can start off by only using them for GUI applications. The most used repo is Flathub: https://flathub.org/