Linux

While I absolutely adore everything rpm-ostree/bootc, I do think operations involving it are relatively heavy; at least compared to what else is out there.

Depending on your (in)tolerance, you might therefore consider opting for something else, instead. Assuming that this list does a considerable job at presenting your options, I'll try to provide input on some of the more mainstream ones:

• openSUSE's offerings. AFAIK, it is lighter. Heck, I'd reckon you might not even be able to distinguish it from its traditional counterpart; Tumbleweed. However, its ecosystem is still very much in its infancy. Hence, I don't recall any of their offerings that don't rely on GNOME/KDE-Plasma. There used to be Project Greybeard, but it's far from lively... As for Aeon (i.e. GNOME version) and Kalpa (i.e. KDE Plasma version), they haven't had a general availability release yet.
• NixOS. I have heard good things regarding how well it does on low-end PCs. However, FWIW, my own testing portrays a different story: I once had an update that resulted in a lot of compilation and my machine was struggling quite a bit. The same machine that handles Fedora Atomic quite gracefully*. Perhaps that was a fluke, but I wanted to point it out. I'd argue nix does handle operations more gracefully than rpm-ostree/bootc on average, though.
• Guix System. Perhaps I'm wrong, but I wouldn't be surprised if this outdoes NixOS in terms of how gracefully it operates on a low-end system. This is mostly on vibes, though.
• Endless OS. If you liked Bazzite, but want something lighter, then this might be just that. It relies on ostree only and thus doesn't have the heavier operations from rpm-ostree/bootc. The goals of the foundation behind it align with enabling low-end hardware. This is reflected in their system reqs. Note that GNOME is still relatively heavy, but you should be served well even on just 4 GB of RAM.
• ChromeOS Flex. For completeness' sake, if you're otherwise okay with this, then I suppose it's another option worth considering. FWIW, there's also FydeOS (and perhaps others).
• VanillaOS. Does something similar to bootc ever since its Orchid release. So, it's probably not that light. Furthermore, I got many questions regarding the health of its ecosystem. This used to be another serious contender, but I'm afraid it might have missed the boat...
• GNOME OS and KDE Linux. While not production-ready yet, these will definitely be interesting in the long run.
• aerynOS. Another project that's not production-ready yet.
• Nitrux. Definitely one of the more interesting ones. It has been around for quite a while now, but I've yet to come across someone that dailies it.

Having said all of that, the gist is basically that atomic distros are still relatively new. As such, I can only recommend Endless OS, Guix System and NixOS. Note that the latter two are rabbit holes, though.