OP, I don't intend to convince you otherwise. I merely intend to share my own takes on this. So, without further a due.

1. Last time I used it, Fedora's updates were too unstable. I twice got updates breaking my system setup.

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'.

2. I don't like Red Hat. Even though I understand that open-source projects are complex and I should separate decelopers from their software, that doesn't change my opinion on Red Hat.

Fair. What makes you hate Red Hat? I know often cited reasons for why people hate Red Hat. But what are your reasons*?

3. This problem stems from the previous ones. Using Fedora I feel like a beta-tester for future Red Hat projects and especially RHEL.

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.

Keep in mind, that I last used Fedora on versions 37–38 and things might have changed since.

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.