I think CentOS Stream, Debian or a tweaked Ubuntu LTS are good for stability and all free also as in freedom (after replacing snap with flatpak on Ubuntu).
OpenSUSE slowroll is a good model for better tested but not randomly held back packages.
Fedora has the older stable release, currently 39. It is more stable than the current 40.
As a workstation Desktop I can recommend KDE Plasma, but it is not bugfree. Plasma 5 has bugs that will not be fixed, Plasma 6 has those fixed but random other bugs and random missing features.
GNOME is unusable in many parts for me personally, but very very likely the most stable but also modern Desktop.
COSMIC will be pretty awesome. It doesnt really have bugs for me, but simply a ton of missing things. But the way they build the project, how well everything works and implements all sorts of "we have this new shiny thing" from various DEs like KDE Plasma, is really nice.
But that will take at least a year to be really finished.