What problems do you have with your lower back? Because it looks fine on the ascent. You are right that it sometimes unlocks during the descent, but that is easily fixed--force yourself to hold it in extension until the bar hits the ground, and drop the weight (much) faster.
Candidly, these look pretty good. Two comments. One, make sure the bar stays in contact with your leg the whole way up. Two, add weight, because this looks easy for you.