I've been using an immutable Fedora for quite a while now and it's amazing. I use toolbox and run everything in containers, including GUI apps, so not even using Flatpak which I'm not a big fan of. I think I have like 3 or 4 packages installed on top of the base image. The peace of mind while upgrading can't be overstated. Highly recommended.
Just installed Onyx yesterday on my work machine! So far so good, some niggles as always (it's my first time using Budgie), but overall fairly usable and easy.