they have done away with OS-tree? Because that would make this entirely based on FlatPaks and no other options, which is a huge block for me.

I don't understand exactly what you mean with that, but I think you are afraid of any restrictions.

• uBlue (Bluefin, Bazzite, etc.) is still Fedora Atomic, just like Silverblue. It's just that they take the OG image, rebuild it based on some instructions, and then redistribute it. It still has OSTree and all other stuff.
• You aren't set on Flatpak, but you definitely should use it on image based distros. Flatpaks are great and convenient, that's why they're getting more and more popular, also for devs. Because of that, the default (and only) way of installing apps via software center is Flatpak. If you don't like that, you can still use Distrobox (e.g. with Pacman, DNF, etc.), Nix, Brew, or any other package manager you like, b but that's more for CLI-users.
• I mostly work graphically, but if I have to do some CLI stuff, then I enter my Arch-Distrobox. I never encountered any problems or restrictions there tbh
• And you can still layer (install rpm packages on the host system) via rpm-ostree if you really need it, but it's not recommended and only there for essential stuff. Use containers instead.