RISC-V chips tend to be slower, but architecture-wise there's not much an ARM chip can do that a RISC-V one can't. Both are pretty well-supported for the coding/web browsing use case laptops get employed for.
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.
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.
You could mod one into the hardware if you really wanted. The drivers for the Magic Trackpad are pretty much flawless on Linux, you could engineer your own plug-and-play solution with COTS hardware if you found the motivation.