It's taken nearly a year, but someone has finally made a much simpler method to install third-party launchers on your Steam Deck, such as Epic Games Store, Battle.net, Ubisoft connect, Rockstar launcher, Amazon games and more.
Huge shout out to moraroy on GitHub for creating the Non-Steam Launchers on GitHub
First off you want to go into Desktop mode on your Steam Deck, open your chosen browser and navigate here: https://github.com/moraroy/NonSteamLaunchers-On-Steam-Deck
Scroll down and hit the download link or right-click and say save as, which will download the .desktop file
Run the Nonsteamlaunchers.desktop file, accept the warning prompt.
Now you can select all of the launchers that you want it to install
Once you have your selections, hit ok and let it run until you get the restart steam prompt.
Contemplate the question and hit ok, Steam will restart and the launchers added to your library.
Open Steam and change the compatibility for each of the launchers you've installed to Proton Experimental by clicking the settings cog under each launcher on Steam, going to compatibility and setting Run this with a compatibility layer.
Stay in desktop mode to link your launcher accounts, this is much simpler especially if you have your browser signed in for google auth etc. You will need to use steam shortcuts or the touch screen to navigate around the launchers or change the controller configuration for each one.
Optional but highly recommend - Download and install Decky Loader from here if you want to easily manage your artwork for the launchers (and other games) directly in Game Mode.
This will need a sudo password but if you forgot it or haven't set one, we have a guide for that here
Optional - We also recommend setting up the auto-mount storage script from here if you want to install games on third-party launchers to your SD Card or other connected Storage devices as this will make sure they are always mapped to internal drives for you without additional command line arguments.
In desktop mode, if you installed Decky Loader, use the 3 dot menu (...) to access the decky menu at the bottom and go to the marketplace icon
Scroll down and install SteamGridDB
Now you can set artwork for your launchers by selecting properties -> change artwork
You will need the Capsule, Wide Capsule and Hero to fill all the images that you see in your Steam Library
Once you run a launcher from game mode, you will need to change the controller layout to make sure it works when you run the games
Select controller settings and change it to Gamepad with mouse Trackpad
Press X on the display screen to apply the layout, now you will have full controller mapping in games that support controllers and mouse control via the trackpad while in launchers, you will still need to use Steam+trackpad click or right trigger click to mouse click though.