Here's everything I had to do to launch the editor on my NixOS (26.05 unstable, native steam) system.
Firstly, manually set both the game and the editor to use Proton Experimental.
Now, (re-)install both.
Launch the game first, then the editor. Hit yes when the game asks you to install DotNET. If either start compiling Vulkan shaders, skip it.
Next, open your terminal and go to your Steam library folder (cd /path/to/SteamLibrary/steamapps/common)
Delete the sbox 2129370 folder (rm -rf "sbox 2129370") and make a shortcut pointing it to the sbox folder (ln -s sbox "sbox 2129370").
Now, go to your compatdata folder (cd ../compatdata)
Delete the 2129370 folder (rm -rf 2129370) and make another shortcut to the normal game's folder (ln -s 590830 2129370)
Now, you should be able to launch the editor. Loading the example project took > 2 minutes and stalled five times on my fairly beefy desktop PC, but it does load and run with only minor graphical issues.
The actual game loads after only like 20 seconds.
