Others have already answered most of this, but I’ll add my piece:

Gnome on SteamOS

It’s possible, but every steamOS update will wipe it. There are scripts out there to automate reinstalling gnome after every system update. Still, Bazzite would be the easier long term option. I personally prefer Gnome as well, but I’ve actually been fine with KDE on my deck. I’m not frequently docking my deck and using it as a PC though.

Easy deck upgrades

You can very easily replace the SSD, backplate, thumbsticks, fans (some of the LCD models have a whiny fan) and rear buttons. Replacing the front buttons takes more disassembly but isn’t too bad afaik. Replacing the front plate or the screen requires separating the screen from the frontplate with heat, which is where a lot of people become uncomfortable.

Many replacements and alternative parts exist. You can get transparent or multicolor shells, shells that claim to offer better cooling (which I’m skeptical of tbh), hall effect joysticks, extra clicky button assemblies, alternative back button designs, buttons with different materials and textures, and higher resolution or OLED screen upgrades for the LCD models.

Other stuff

If you primarily play on PS5, be sure to check out Chiaki4Deck as it will let you stream your PS5 to your deck. If you get the OLED model it will even support the PS5’s HDR, which is a benefit you don’t get with the official PS Portal device.