I also use ffmpeg to trim the video, using a command like
ffmpeg -i my_video.mov -ss 00:00:20 -to 00:08:40
-c:v copy -c:a copy my_trimmed_video.mov
(to discard the first 20 seconds and everything after 8 minutes and 40
seconds). You might also want to cut some sections out of the middle
of the video, for example where you are erasing and cleaning the board.
That can be done with ffmpeg as well, but it is more complicated. I
am working on some software to streamline all this. However, all these
operations are fast, they only take a few seconds.