Just to clarity the relationship between Red Hat, IBM, and Fedora, Fedora is only sponsored by Red Hat. They make all their own decisions, and while they receive financial support from Red Hat and Red Hat owns the Fedora trademark, their decisions and development are independent of Red Hat (and by extension IBM), with the single exception that they cannot risk violating the law (i.e. copyright infringement), else it risks Red Hat legal trouble (and Fedora would risk losing their sponsorship as a result). Red Hat benefits from Fedora’s development by the community, given that Fedora is RHEL’s upstream, hence why it continues to sponsor Fedora. But it isn’t Red Hat that is in charge of Fedora’s development, it’s FESCo, which is entirely community elected, and does not stand for the interests of Red Hat, but rather for the interests of the community.
Eliminating Fedora from contention in that regard is essentially like eliminating Debian because you don’t like Canonical, who makes Ubuntu, a downstream of Debian.
Add on top of that the fact that IBM and Red Hat are major contributors to the Linux kernel, and you absolutely cannot avoid connections to them while using Linux. I mean, that’s quite frankly a ridiculous exclusion criteria in the context of Linux. If you’re looking to avoid an operating system OWNED by Red Hat or IBM, then Fedora should not be included in that list. Neither of them have any say or pull in the development of Fedora, which is a completely community-driven project (no, owning the trademark doesn’t change that fact; if Red Hat tried to take over, the Fedora community would simply fork the project, rebrand, and continue on their own). Besides, Red Hat has no interest in controlling Fedora, because it doesn’t benefit them. Their only interest is in enterprise applications, which is not a good use case for Fedora. The only operating systems Red Hat actually has any control over are RHEL, CentOS, and any derivatives of those operating systems like Rocky Linux, Oracle Linux, and such (though Red Hat’s control over derivatives was only the result of those projects being downstream, not actual ownership).
So with that in mind, I’d recommend the Fedora KDE spin if you want a normal, stable, snap-free, no DIY required distro with KDE, or if you want the immutable version, Fedora Kinoite is what you’d be looking for. And Fedora has the major advantage over Debian-based distros of actually receiving package and kernel updates regularly, so you can stay up to date and enjoy new features, all while maintaining stability.
Fedora Kinoite is absolutely the best immutable distro fitting your criteria. Anything else will have a much smaller community and less support as a result. rpm-ostree has great documentation, and all of the Fedora Atomic Spins have a huge userbase available in case you ever have questions.
Excluding Fedora because it’s “too close to RH” doesn’t make any sense at all. Fedora is not controlled by Red Hat, and Red Hat has no interest in a consumer desktop platform that they can’t sell. Fedora’s development is managed by FESCo, a community elected board that represents the interests of the community. They are kept intentionally separate from Red Hat’s development, and don’t tailor their development to Red Hat’s wants or needs (in fact they often do the opposite, as Fedora pushes for change in the way things are done, not stability, as can be seen by the exclusion of X11 from Fedora 40, for example). That stands in direct contradiction with RHEL’s goals. The features that are pushed by Red Hat developers would not be approved if they stood against the wants of the community, so anything Red Hat does contribute benefits the community as well. Red Hat’s entire business is in enterprise solutions, as their business model relies on them selling support for their software. There is exactly $0 in potential revenue from Red Hat trying to take over Fedora, it just doesn’t make sense. They can’t sell anything, and since Red Hat doesn’t employ all of the thousands of active contributors, such a takeover would simply result in a new fork. In fact, it would be against their interests, as Red Hat actively benefits from the developments of the community. Taking over control of the project would lose them all of the constant volunteer work put in by the community, which costs far less for them to sponsor than it would to employ a team a fraction of the size on salary. I’ve discussed this topic at length many times before, so I’ll just link to a few comments that explain the situation in more detail (including how the project is funded, managed, and separated from Red Hat).
https://lemmy.world/comment/7490965
https://lemmy.world/comment/7494803
The best fit for your criteria is Fedora. If you want uBlue spins, you’re still getting Fedora, just a more opinionated version. All of the major development of uBlue’s images comes from Fedora though, as they don’t maintain their own distro, they just repackage Fedora.