Maybe this is something I can give a try. I could re-purpose my 5600x mITX current build but put it in a much smaller case.
Are you using a dedicated GPU in your Steam machine?
Yes, it's basically my old PC that I bought in 2022, but with some new bits added after I upgraded to the AM5 PC in June. Originally it was a 5600X, 32GB DDR4 and 6650XT in a Corsair 220T case. So basically the same as Eurogamer's 'test' comparison PC spec when they did their Steam Machine coverage. But after getting that annoyed with some of Sony's recent malarkey I sold the PS5 Pro and used that money to upgrade the 5600X & 6650XT to a 5800X3D & 9060XT 16GB combo. That also freed up a 2TB WD SN850 from the PS5 Pro which I used to upgrade my storage in the AM4 PC.
I have tried adding a second NVMe drive, but gave up because it isn't easy to do like on normal Linux. You can fudge a solution, but it's 'hacky' so in the end I just used an external USB-C drive for storage (which tbf has been more than fast enough). Ideally I would downsize the motherboard to mATX and transplant into a smaller case. That said the PC is tucked away on the sideboard so I can live with it although it's no where near as nice looking as a small form factor PC.
I had used Steam OS before on my Steam Deck and (before it went official OOTB for everyone) on my old Minisforum miniPC (AMD 4800H APU). I think the nice thing about Steam OS is that as long as you have suitable hardware (i.e. anything AMD, not tried Intel) you can freely try it. It's not perfect and there are times I'm occassionally reaching for my Logitech K400r keyboard. I wouldn't currently run it as a desktop OS though. The immutable OS doesn't work for me atm; lack of things like being able to edit the fstab file, or edit my Pipewire (audio) config amongst other things just limit it vs. a fully fledged distro like Fedora, or Arch.
However the great thing about Valve's work on Linux; is most of these solutions like Game mode, Proton etc. are FOSS by design and you can use in any combination you want to.
EDIT: Forgot to add - dual booting - I wouldn't do it, but that's me. I did try it once on separate drives, but I had issues. Particularly with Windows removing the boot loader. You can work around this and try and limit the configuration. But it's full of issues. Ideally if you have a way of separating your Windows and Linux PC's I think that's a far, far easier life. Again Steam OS lends itself quite well to this in that you can have a second PC in the lounge running Steam OS.