You can set BPM to launch at boot just by going into Steam -> Settings -> Interface tab -> Enable “Run Steam when my computer starts” and “Start Steam in Big Picture Mode”
I also have two other Samsung TVs with their Tizen OS thingy that has Steam Remote on them so I can log in and play any game remotely from anywhere in the house if I want. It's pretty crazy and impressive, considering that as a kid, growing up it broke my heart that I can't even play the Rayman 2 demo I had because I couldn't afford a 3D accelerated GPU :)
Using Windows on Steam Deck (not using Steam on Steam Deck, using the Windows OS) is going to be difficult because the device lacks a mouse and keyboard on which Windows is dependent. Fixing this for Xbox required making an entirely new UI (for the OS, updates, sign-ins, everything) which Steam has not done for the Deck.
Not coincidentally, this is the same reason that MacOS doesn't have touchscreens. Everything about that OS also requires a mouse (or touchpad) and a keyboard. Just adding a touchscreen to a Mac won't do it.
It seems to me that if anyone really wants to do power user stuff on it, they'll dock it and use a keyboard and a mouse anyway on a far larger screen.
The point I'm making is that normal folks will not dock it or goof around with touchscreen compromises in order to run Windows, which they will also have to install themselves! This is the opposite of a smooth, integrated, pick-up-and-play experience.
Since MS makes hardware now, what's to keep them from making a handheld that runs the Xbox version of Windows?
A Microsoft handheld would run less games that a Steam Deck with Windows because console exclusives like Horizon Zero Dawn are available on Steam but will never be on Xbox. An Xbox device also wouldn't support emulators or someone's existing Steam library.
I don't know to what extent it would be worth it for Microsoft to dabble in that area. PlayStation Vita didn't work out so well for Sony.