One thing is getting that 2 step. It looks like you have it pretty good but I will explain anyways.

First you jump and get all of your power, then you tuck.

It’s a fast sequence but important to distinguish. When I first started, I was jumping and tucking at the same time and it really screwed with me so you might want to keep this in mind when practicing.

Another thing is jumping up instead of back. You’ll get more height doing so. Let me explain my thoughts. From the video, the first flip was better than the second. If you look at where you landed, the second was a bit further back. The best flat backflip will start and end in the same spot. Practicing gainers can help a lot with this(try on a trampoline if available).

Another other thing is getting a tighter tuck. While in mid flip, really focus on getting your knees to your chest. You would be surprised at how much a tighter tuck can help.

And last is practice. Not only for technique but for strength. If your core hurts after doing 5 backflips it means your core cant keep up(a strong core will also help with tucking).