SteamOS is an immutable Arch. Valves aim is to reduce support costs by ensuring everyone has the same build and to only support a hardware subset (AMD APUs) so it's less general purpose than a regular Linux distro intentionally. A steam deck is just a PC though and it is usable for non gaming tasks the same way a gaming focussed immutable system like Bazzite is. I even did development on a Chromebook for a month or more years ago as a challenge. It's possible. It wasn't ideal. The further you get from steam hardware and use case the more hoops you will need to jump through.

Games generally run more or less the same on any Linux system if they have the same kernel , steam runtime, mesa and proton in my experience. CachyOS might get a few more FPS until the patches they use get more widely distributed. Some compositors will get a little more performance than others.

Some games have detection for Steam deck that works around bugs they haven't bothered fixing for proton users in general. I have one game I had to set an environment variable so it would behave like on steam deck.

I think SteamOS on a mini amd apu system hooked up to a tv as a gaming system would make a lot of sense. Running it on a regular desktop for non gaming taks is more of a novelty thing. It's less practical than using a more general distro.