Hey Michael,
I wouldn’t use a metronome along with the video. As you said, it will be very hard to match the timing perfectly. If you are playing along with the video, then that will act as your metronome, as I am playing to a metronome myself. However, when you are practicing alone, you can set a metronome, and that will help you stay in rhythm. For Amazing Grace, 120 BPM is pretty standard, but when practicing for the 1st time, it’s good to slow that down even more, maybe to 80 BPM and work your way to 120.
I use the guitartuna app. It is a great tuner, and it also has a great metronome feature too.
You never fail until you quit!