Incidentally I made a vehicle based game in VR (https://roguestargun.com), but currently I have no idea how to upload the APK to Steam to run on-device for the steam frame.
https://partner.steamgames.com/doc/steamhardware/steamframe/...
Potentially related, IIUC Unity only has linux arm64 support for the embedded linux version. And to use that you have to contact sales.