Your lack of lumbar extension is the biggest problem, and at this point, it's the only thing you should be thinking about fixing. Unfortunately, this kind of thing is hard to do online, it would be much easier to fix this in person.
First, ditch the belt.
Are you able to set your back in and out of extension while standing, like demonstrated in the lower back positioning video? Are you able to do the same when bent slightly at the hips? With both the hips and knees bent?