I'd suggest increasing the gamma on the video if you can, like you say it needs more exposure, although 20 seconds at ISO 800 and F/2.8 should produce a brighter image than you have I think. My recent timelapse of Orion was 800ISO 25 second, F/4, and was significantly brighter. Perhaps RAW and post processing makes the difference.
Those last few seconds are nice. It would have been good if you'd kept going a bit longer I think, as the video stops with the tail and stars still visible, it'd be nice to captalise on the time you have the object.. get a few more seconds maybe.
Good on you for giving it a shot
