-I love Gnome, but I’d be willing to give KDE a shot. If I don’t like it, how difficult would it be to have Gnome while keeping the normal Steam OS?

Afaik, not possible. SteamOS uses a mostly immutable A/B partition structure, so while you could likely install Gnome, the next time SteamOS updates, I expect it would wipe out your tweaks (or enough of them to break shit).

-I know that I could wipe Steam OS and get Bazzite with Gnome. Except getting Gnome, what are the advantages of getting Bazzite over Steam OS? What are the inconvenients?

Pros:

• It won’t wipe out your system tweaks at every update.
• It will use the most updated versions of things available in Fedora’s repos.
• A bad update can be easily rolled back.

Cons:

• It’s not like traditional Linux distros, like Workstation, so learning how to work with an atomic distro can be a challenge at first.
• It might not have every patch SteamOS has, or at least not right away. This tends to be minor in practice, but Valve has a vested interest in making the Deck awesome, not Linux as a whole. The community is pretty good at keeping up with downstream patches, though, if any.

Noteworthy:

• It uses Wayland by default. That matters to some.
• Some software cannot be easily installed (like some VPN clients) or installed at all. The more a program has to touch system files, the less likely it is that you can install or use it.
• Updates take a long time due to how the snapshot imaging works.

-I think that KDE is now in version 6.3 or 6.4. What is the version that you now have on Steam OS?

5.27.10. SteamOS uses X11.

-What can you easily upgrade on a used Steam Deck (probably not Oled)?

For the LCD version, lots. From the screen to the shell to the buttons. I vaguely remember there being a fan mod, too. There’s a whole ecosystem of Deck mods out there. You can even remove the screen and controllers entirely, if you’re so inclined.