If you need a virtual monitor for your Quest under Linux, you can use Immersed https://immersed.com/. It is not FOSS but has a free as beer option. It’s just a regular app on the Oculus app store, no need to switch to developer mode, no need to install Side Quest, no need to build from source code.
You can use the USB-C direct connection to minimize input lag; in this case, you will need to switch to developer mode, and I think it will also work in offline mode. I did not test that scenario though.
I have a Quest 2, and sometimes it is nice to lay down in bed and do something on my laptop without needing to go to my desk. Also, I am thinking of trying this setup outdoors (camping, for example).
It does work with SteamDeck; I have a USB-C hub, so I can have a direct connection and charge Quest and SteamDeck at the same time. I want to see how gaming is going to be on that setup.
I want to mention that it is only for virtual screens; it does not have integration with SteamVR. So if you want to play PC VR games, you need to look at other options like ALVR.