Use what distro you like, but most distros are very easy to install (some even easier than Ubuntu I would argue). KDE Neon would be a good starting point in that regard. What exactly is hard to understand about Plasma? I have heard this sometimes now but I really don’t get it, I find it to be very easy to understand as it integrates for example theming
It should work on Mint as well, but the theming may be off you will probably have to redo Icons and such. Just FYI, XFCE can totally look like Gnome (look at rhino Linux) if you theme it. You may consider installing the SaveDesktop app and redoing your XFCE setup (the unixporn community has some great inspiration)
macOS and Linux have additional security features at a system level, on Linux most software comes through controlled repositories or sandboxed flatpaks. There are also tons of multi million dollar companies that constantly try to find and fix kernel level vulnerabilities and a distro like Debian, which is very popular for servers, has had less major vulnerabilities than windows 7 throughout its entire lifecycle and Debian exists for other 30 years. So I’d say Linux is would have a few less (different) attacks