Nobara Linux. The official website is WIP but it’s essentially a distro aimed for gaming, with the important things like Steam and Lutris pre-installed.
I'm currently using it and it's pretty good and reliable. It defaults to wayland though, you may want to go back to Xorg if you're using an Nvidia gpu.
I tried Nobara and really liked it. Some games didn't like my the hybrid graphics on my laptop tho and I ended up swapping to Pop. But I actually like Nobara better than Pop and would be using it if I could.
Newer GPUs are fine in Wayland. I have a 3090 and Wayland is the only way I can get full refresh rates on all my monitors because X locks you to the lowest value. Only thing is that the translucency on themes doesn't render super well and sometimes the system clock and taskbars will freeze up. That aside, it's been a good gaming experience overall.
HoloISO if you -REALLY- want to have a "steamdeck-like" experience. Unless if you mean "steam focused" as in "gaming focused"....? Then (pretty much) any distro you want -- as long as its a minimal install. Tweak it to your needs, make steam run when you log in. And thats it.
If you want minimal effort to get a good Linux setup for Steam, just buy a SteamDeck. Get the dock if you want to use it like a regular computer or console with a wireless gamepad. I did that -- hooked it up to my monitor, headphones, plugged in a mouse, keyboard, and my old XBox360 USB wireless dongle and it all just worked. I've got a few ideas for fun projects I want to try with it as a handheld and have written some software on it using desktop mode (little Python utility scripts for shuffling data around) but mostly I just use it like a gaming console; it works well for that.
I use Ubuntu for my gaming PC. I use it like a PS5 or XBox- I turn the PC on, it auto logs in, auto runs Steam in Big Picture Mode. I use my Xbox controller to select and launch the games I play. I really have not turned on my PS5 in months. And games I was considering buying for the PS5, I will get for my Linux PC. I even add non steam games to my Steam library and play them with Proton/steam.
Currently playing Baldur's Gate 3 and Lords of the Fallen. Both on maxed settings. Looks and plays great.
Or hear me out, Op is building something that would need ease of use or plug and play experience. A homemade Steam Machine would benefit from a Steam focused OS. You are just being an asshole.
Don't let this guy discourage you from trying it. I've been using Linux for over 20 years, and the Steam Deck is the most polished experience I've seen for gaming on Linux, even for non-Steam games. If you decide it's too limiting, you can just switch to something else.
u want your whole os around steam and I'm the 13 year old? stop projecting bro. and your eyes hurt from rolling? may I suggest some sunlight, gaben enthusiast?