I've said this dozens of times: IME perhaps HALF of the reason why cycling hurts me is bike fit. everything else is my body conditioning. most cyclists have poor core strength and tight hamstrings. no amount of perfecting your bike fit is going to help when that is the issue. my lower back pain goes away when i start doing planks, bridges, russian twists, deadlifts, lunges, squats, etc. consistently.
your saddle is likely too high and that handlebar looks HUGE (long reach). start with that.s