Great distro ! I have been using it for the last 2 years on my Framework laptop 16 without any issues. I even have a "fork" of sorts that adds Hyprland + all of my "desktop" config, which I think as being part of the OS.
I really think immutable distributions are the future of linux desktop, and maybe distributions that use OCI images, beacause they are a lot easier to work with than say, NixOS for example.
If you want to have your custom bazzite, you just do a "FROM bazzite:<whatever-version-you-want-to-pin" and add stuff you want.
Of course, you loose a bit of the reproducibility, since usually container images do not pin packages (and maybe other reproducibility issues I am not aware of) but it is way easier to work with.
Immutable is very good for new linux users but I personally don't like the restriction and find rpmostree extremely slow to install literally anything. It does make sense to use immutable distros in routers, firewall, etc
I'm an https://getaurora.dev user and I agree uBlue is awesome. I'd like to create a custom image too, but it doesn't seem quite as easy as you say: https://youtube.com/watch?v=IxBl11Zmq5w