Watching your vids you can see before you start both movements your lumbar is already in extension, you actively need to tilt your hips back and start with a much more neutral spine.
The only advise I would offer additional to mrthingyx would be to stop doing ab rollouts and focus on movements where it's easier to control your lumbar spine. Rollouts are quite an advanced movement to get right, and there's a lot of opportunity to get them wrong and they can very easily emphasis lumbar extension and so would be best to avoid give your natural stance to be in quite a lot of lumbar extension already.
Also front squats, get those elbows up more, when you put more weight on that bar is just going to roll off you at the bottom. However they do look good.
The only advise I would offer additional to mrthingyx would be to stop doing ab rollouts and focus on movements where it's easier to control your lumbar spine. Rollouts are quite an advanced movement to get right, and there's a lot of opportunity to get them wrong and they can very easily emphasis lumbar extension and so would be best to avoid give your natural stance to be in quite a lot of lumbar extension already.
Also front squats, get those elbows up more, when you put more weight on that bar is just going to roll off you at the bottom. However they do look good.