Another method is to save the config as a template. The whole situation is so frustrating because before the switch to the Deck UI there was another tab which listed your configs for other games. I hope they bring that tab back at some point!
The Shield was made for game streaming which would precisely enable you to connect the controllers to it and run the games on your PC. Nvidia has since stopped supporting that software but luckily there are alternatives. The Moonlight client and Sunshine host are direct implementations of what Nvidia used to provide or there's Steam Link if you're using Steam.
I know the DS4 controllers had similar connectivity issues on Windows which could be solved by changing power management settings. I'm not sure if this issue still applies to using DualSense on Linux.
You can see some hardware information at https://store.steampowered.com/account/hardwareused. There's an example here: https://www.xda-developers.com/how-find-steam-deck-hardware-specifications/.
It's such a shame. I really hope they make it editable at some point!
The closest I ever got was just using two controllers. I really hope Valve will make a new controller for the Steam Deck and it's able to be split in half.
There are lots of useful programs that can be installed through the Discover Software Center as flatpacks. Some of my favourites are Gimp, Kdenlive, Kodi, Blender, LibreOffice, CoreKeyboard, Discord, Flatseal, KDiskMark, MComix, Calibre, and Pidgin.
Some fun ones are Space Cadet Pinball, Prism Launcher (for Minecraft) and Heroic Games Launcher (for GOG, Epic Games and now also Amazon Games).
I agree that this needs to be an option but unfortunately I haven't seen any solutions either.
I don't think there's a significant performance difference but I haven't tested it. One difference is that Desktop mode still uses X11 whereas Gaming mode uses Wayland. That can effect some programs but I don't think any games mind which is used.
Last time I checked Shift+Tab would bring up the overlay in Desktop mode but nothing worked in Gaming mode.
I really like this Shiekah Slate one: https://steamdeckrepo.com/post/nv4lN/botw_shiekah_slate_boot_video