Install steam. Run in big picture mode. Done. That's a steam machine. I don't get what you think a dedicated machine is going to do any differently. There is a reason Steam abandoned the idea themselves.

Big picture mode on my windows PC and the gamescope-focused UI on the Deck look similar, but offer very different capabilities IME.

To name a handful: FSR support for all games - including those that don't support it, per-game hardware performance profiles, excellent hardware integration - not just limited to the instant sleep and instant wake. With the third party Decky Store you can also configure the fan profile to your liking, control music apps running in the background on the Deck, and more. On the PC BPM these sadly do not exist

I 100% prefer playing on the deck any day of the week - the OS simply makes it so straightforward to jump into a game and forget about needing to also think about maintaining a desktop: no Windows updates, no telemetry service CPU spiking, and no Windows resetting my customized settings or forcing Edge browser defaults after an update.

That said, I don't particularly have an interest in a full blown Steam Machine - for me the Deck works just fine when docked.