I recently got rid of my M1 iPad Pro, so I can't verify (and you might be right).
I recall: using stage manager, plugging into monitor via USB-C, going into Settings to disable mirroring/enable as external display, ensuring you have a mouse/keyboard connected to the iPad, and then just clicking the iPad screen off with the lock/unlock button.
The fact that you can't turn off the iPad screen while it's docked drives me crazy.