As is you can buy a decent AMD mini PC for about $500 and just install Linux on it. It works very well for the most part, and a few distros offer steam OS like experiences.
For my mini PC I couldn't get the EGPU to work with Linux so I'm stuck on Windows for now... But I play a few games that are Windows only ( anti cheat) so this is for the best.
Ideally what you'd be paying for is Valve's first-party partnership, and therefore a commitment to tailor Proton development to specifically ensure that this hardware keeps working (at least as well as a Steam Deck works, anyway). I believe this is what Valve has done for the Lenovo Legion Go S.
That is ideal, but also pretty unnecessary. The only thing AMD has to support on their end is Vulkan, and the work on that front is effectively finished. What Valve can offer is HID support for handheld hardware and potentially shader caching servers for huge swaths of identical hardware models.
With a desktop there's a limit to what Valve can commit to. There's not a single controller firmware to support, and probably not even a consistent GPU setup to cache for. The extent of realistic support for these AMD boards is kinda fully realized at this point. Proton is, and will remain, a plug-and-play experience for AMD users that own supported hardware.
The "support" is complete. Proton has only a few critical dependencies and they are officially supported by AMD's GPU drivers already. I cannot name a single part of their hardware stack that would not get supported on-parity with the Steam Deck.
Valve as a company could shut down tomorrow, and AMD users could still use Proton to play Windows games as long as their GPU is Vulkan 1.2 compliant.
For my mini PC I couldn't get the EGPU to work with Linux so I'm stuck on Windows for now... But I play a few games that are Windows only ( anti cheat) so this is for the best.