I have used the OTG to Easycap method to record my DVR footage for my first year. It did drain battery fairly quick, equal to watching an HD movie on the phone. But not in minutes, with a fully charged phone I've always been able to record my full sessions (around 8 packs).
The Easycap viewer app supports stereoscopic view (for a few dollars extra) for use with a VR headset, I have tried flying with it once but the latency was noticeable. What I did love was the DVR quality. It's better quality than fatshark built in DVR, it's 60 FPS and you get the audio of your phone in the recording so you can hear the motors.
But I got tired of fiddling with the wires, freezing of the app because a cable got loose, notifications popping up on my phone right when I did something epic (lol darn!!) so I'm using the Fatshark goggles DVR now for the convenience. But quality wise, the Easycap setup was better.