For what it's worth I have a ROG Ally X with both Windows 11 Pro and Bazzite both installed. It came with Windows 11 Home but I upgraded windows to get more access to things like Group Policy Editor specifically so I could turn off pretty much everything MS put on it that I didn't want. Local Account. No telemetry. No ads. No tacking. No copilot or AI. The only handheld that comes with Steam OS right now is the steam deck and the Legion Go S. All the others come with Windows.

For the purposes of using steam, windows isn't inferior exactly but I don't like it as much. Touchscreen support for Windows 11 (os-wise) is pretty poor in comparison to previous Windows distros like 10. You can limit but not completely prevent other processes from running in the background in windows. But once you launch steam big picture mode it's basically the same. There's very few functional differences (hardware things mostly that are handled by other apps like RGB lighting or fan curve and TPM and battery charge limits).

How windows performs is ultimately dependent on the hardware and that's a lot of the reason that I can't speak for how windows would perform on a lower specced device. I doubt windows on a steam deck would perform the same as steam os on a steam deck.