i did some reverse engineering of #quest3.

it is possible to completely disable existing ui. in this state where you see only black, it is possible to launch immersive application.

disable default ui:
adb shell pm disable-user com.oculus.vrshell

start immersive app, such as immersive custom launcher:
adb shell am start -n aaa.QuestAppLauncher.App/aaa.QuestAppLauncher.App.AppInfo

note that controls do not work except centering the view using long press of home-button.

some other commands that may be useful:

hide task bar etc:
adb shell pm disable-user com.oculus.systemux

re-enable full iu:
adb shell pm enable com.oculus.vrshell

hide windows, (background and controller remains visible) ?? may or may not work
adb shell am force-stop com.oculus.explore

start lightning launcher:
adb shell am start -n com.threethan.launcher/.activity.LauncherActivitySearchable

hide windows, (background and controller remains visible) ?? may or may not work
adb shell am force-stop com.oculus.explore