DJI FPV - so far I'm flying with video transmission settings for drones and glasses. Would I save battery power for a longer flight if I only recorded video to a drone? And I wouldn't lose the last 30 seconds. in case of accident ???

I don't think you would save anything that you'd notice. And only about the goggles battery. The drone transmits to the goggles anyway. And that is what the goggles - not the drone - records...