I was going to post for help yesterday but solved my own issue. Figured I'd share for anyone in my same position.
I've been trying for a long time to use MRUK, without any success as the app would never load my saved room data when launching the standalone app (using over quest link in play mode would work).
Turned out you need to have a developer organization name setup for MQDH, and also "create new app" with the exact same name as the name you give the project in Unity (under player settings). Then when you deploy the apk build to the device it will automatically sign it with your organization key and therefore "trust" the app. Otherwise it's an untrusted app, which Meta doesn't allow access to saved spatial data. There's no obvious warning messages related to this that pop up, so it's something you just have to know.
