Thank you for all your replies!
I really wish Apple would continue to provide security updates for MacOS High Sierra, but they aren't... I'd much rather stay with MacOSX than move to Linux, but I need to know that my Mac Mini mid2011 is getting security updates.
I'm leaning towards Ubuntu because it seems like the Linux distro with the biggest community.
KDE neon (a rolling release based on KDE on top of ubuntu LTS) worked quite well on my 2011 mini (when I still used it). If that is the way you go, you should be quite happy.
Even though it would be nice, unfortunately Apple does not release security updates for anything older than Big Sur (at the moment).