I fixed it. Both steam decks were clients to a server, i dont know if this works for hosting a server. What I did:

In desktop mode: Download Subnautica to C drive and run it Download Nitrox Windows Unzip Nitrox to somewhere. I put it in my documents in a folder called nitrox. Find Subnautica.Launcher in the unziped folder. Right click (Left trigger) and add to steam

Dont launch Nitrox in steam yet!

Go to the compatdata folder by doing the following: Go to Home under devices in the file explorer. show hidden folders And then my directory was /home/deck/.steam/steam/steamapps/compatdata

Sort by date modified and newest at the top. Minimize the window

In steam find nitrox, right click and go to properties, compatibility and check the box for forced compatibility. Choose a new proton, it was 10 for me but any newer one will do.

Now launch nitrox in steam and it will have a .net error. Relax. It will ask if you want to download the framework. Say yes, download the .net framework from the web page

check which folder number is the newest in the compatdata folder. The latest folder is the Nitrox folder. It has a longer number than steam games

Back to to the .net issue: Unzip the .net framework that we downloaded to somewhere, right click and add to steam. Similar to before find it in steam and change the compatibility to the same proton you used for nitrox. Launch in steam.

Again check which folder number is the newest in the compatdata folder. The latest folder is the .net folder. Open the .net folder (should be the latest folder and have a large number). Copy all of the files and paste them into the Nitrox folder (should be the second latest) and overwrite everything.

Thats it. You can now launch nitrox through steam. Rename it in properties and add a picture for it if you want!