Thank you for sharing the video - it is very helpful to be able to see what happened.
Based on what I saw, I think there are two things at play here (it could be something else depending on your answers to Ecoops questions):
It looks like you were above the glideslope for most of the approach, and finally intersected it at 780-800ft. When that happened, the AP tried to stabilize on the glidescope and ended up overcorrecting, which is why it pitched up, down, up, etc. and ended up slamming the aircraft into the ground. I don’t know why the AP is so overzealous (you can vote for improvement here Smooth Autopilot Transitions), but it is key to make sure you engage APPR when you are already on the glidescope to prevent this from happening.
You were probably going 3-5kts slower than would be ideal during that approach based on how much the nose was pitched up throughout. That created a condition where and inputs to the controls (like the pitch up from the AP) necessitated immediate and aggressive changes in throttle input to maintain lift, which in turn led to more overcorrections. It would have been fine without the AP mixup, but in the future you may want to give yourself a little more buffer to play with.
Hopefully that is helpful. Good luck with future landings, and I’d encourage you to give manual landing a try! It is much more fun when the AP can’t mess it up for you!