Currently, the only projects I'd refer to as (remotely) GA are ChimeraOS, Endless OS, Fedora Atomic, Guix System, NixOS and their derivatives. The rest is, unfortunately, simply not there yet. The closest to these would be openSUSE Aeon. But, if you'd like FDE on your device, then you'd have to forego it for now. Currently, I would advice against relying on any other projects; including Arkane Linux, AshOS, blendOS, carbonOS, MocaccinoOS, Nitrux, openSUSE Kalpa, rlxos and Vanilla OS. Unless, you're fine dealing with whatever random and fringe issues you may have to face.
As for the previously mentioned GA 'immutable' distros, you don't like to pursue Fedora Atomic, NixOS and their derivatives for IMO fair reasons. ChimeraOS is primarily an OOTB console experience distro (aka couch gaming) that happens to be 'immutable'. Therefore, bending it (to become your distro for general use) will definitely be an involved process. But, it's possible. Likewise, Endless OS is somewhat locked down (beyond what you'd expect from your average 'immutable' distro) and has to be bend (at least slightly) in order for it to be more suitable as a daily driver.
This leaves us with Guix System. IMO, if you want to pursue this right now, then Guix System is simply the only remaining way of going forward. It's fit to suit whatever needs you'd have and offers access to official documentation that's at least a decade ahead of the one found for NixOS. However, don't expect this to be entirely painless; 'immutable' distros require (in general) a bit more know-how compared to traditional distros. And within the 'immutable distros', Guix System and NixOS are uniquely positioned for how 'powerful' they feel compare to (literally) any other distro. But, with great power comes great responsibility. Hence, you should definitely know your shit.
Finally, if FDE is not a hard requirement for you and if you can live with GNOME and if don't have qualms against containerizing everything and if you don't intend to tinker, then you might also consider openSUSE Aeon.
OP, I don't intend to convince you otherwise. I merely intend to share my own takes on this. So, without further a due.
So, first of all, you seem to think that Fedora's updates are equally "unstable" compared to those found on Fedora Atomic. But this is simply categorically wrong due to Fedora Atomic being (as it's name applies) an atomic distro. And hence has far superior updates (in terms of 'stability').
Secondly, I recall this period quite vividly, and I actually agree with you that Fedora's handling was a mess. And, unfortunately, this mess also affected Fedora Atomic. Thankfully, uBlue's team ensured that the issues were not felt on any of its images. So, even if, at times, issues spill over to Fedora Atomic, users of uBlue images will not have to face those. Heck, history has recorded that the uBlue images have consistently prevented those issues to spill over to its images. Thus, while this may (perhaps rightfully so) make one question if they should use Fedora Atomic or not; this, however, does not represent the situation over at uBlue images. Hence, one could rely on those without fearing issues related to 'stability'.
Fair. What makes you hate Red Hat? I know often cited reasons for why people hate Red Hat. But what are your reasons*?
Is this specifically a problem because you hate Red Hat? Because, quite frankly, the same somewhat applies to openSUSE and SLE. But this doesn't seem to bother you.
Excellent point. Since that 'double trouble', it has been relatively stable. However, I wouldn't be surprised if Fedora would act similarly if a new issue arises.