It is randomly frozen as not all developers follow Ubuntus release schedule. They just release when it is ready.
Stability means backporting tons of bugfixes to tons of small packages and libraries. I dont think Ubuntu does that for enough packages, best example Plasma 5.27 on Kubuntu. I have reported over 200 bugs I guess and most of the newer ones are just fixed in Plasma 6.
Flatpak for sure is a good way, and if a distro is stable, they should only install Flatpaks.