The raspberry is mainly for old vertical arcade games and a few retro console games. I think most of the games I've got there are MAME emulated. There's this custom Linux distro RetroPie that's tailored for emulation. It has a fancy frontend for launching the games.
Swapping between the RPi and Steam deck is done manually. Actually its a pain, because I need to lift the monitor slightly off the chassis and fiddle with the cables in side. As for the games, Steam deck is the clear winner. There are many modern games even on Steam that can run vertical setup. Mostly pinball and bullet hell type of games, but Its cool to run something like Downwell with this setup. Oh yeah, the egstarts controller board is recognised as a xbox controller on the Steam deck, so its easy enough to map the buttons for each game as needed.
3D printing, eh? I made something similar from wood. I didn't know you could use steam deck as secondary screen for the scoreboard, I should look that up...
Tetris for Gameboy had me hooked. That's why I can't understand why it is missing from this collection. Also no Tetris DX and SNES Tetris? I think I'll skip this one...
I've had the Deck for 10 months now. Mostly I tend to play in handheld mode, but I do play alot in "couch mode" too. I have the following controllers at my disposal, all these work with the Deck:
Steam Controller
8Bitdo NES30 Pro
Nintendo Switch Pro Controller
Logitech F710
The Steam Controller is a bit meh, since it only has one analog stick and no D-pad at all. It's good for games that require mouse control, but other than that I only use it for couch multiplayer games when all other controllers are taken.
8Bitdo is good for D-pad games, so mostly emulators and couch multiplayers.
Nintendo Switch Pro Controller This is the most comfortable gamepad that I currently own. I would use it more on the couch, but usually it is paired with the Switch and it is too much of a pain to change the pairing over to the Deck.
So my most used controller on the couch is the F710. If it is currently working for you, no need to change it. I would wait for Valve to release an updated version of the Steam Controller and get that when the time is right.
UFO 50! Quality fun for docked couch multiplay. Just played a few rounds last night and had a blast.