The universal blue distros are the fastest I’ve been able to have a usable computer up and running and doing what I want it to do. They are fantastic.
There are no official cosmic variants anymore, but there are things like Origami that you can rebase to, if you want to try. Can’t vouch for their stability, but it’s an option. If support is dropped you can rebase back to regular bazzite. Rebasing is easy and pretty safe, it basically acts like an update and switches out the system files, but you should back up your config files just in case the different DE’s don’t play nice with each others config settings. From what ublue developers have said this can cause problems or annoyances.
Or you could develop your own derivitive with bluebuild or something. I’m not sure how involved that requires you to be, but it’s probably easier than learning nixos.
As far as I understand it, the immutable fedora-based distros want as much as possible to be installed at the user level. Layering things onto the system should be reserved for things that need system level access to work properly, like maybe a driver or something like that, which might not have been included.
It’s a better practice for stability and security.