leave your lumbar region alone. it isn't supposed to be mobile, it is supposed to be stable.
ankle mobility and hip mobility for sure. thoracic spine mobility, too (to help with your bench arch).
try holding a light plate out in front of you (5kg) and squat. it will help balance you so you don't fall on your butt (you might need a heavier plate - try a heavier plate if you fall on your butt lolz). when you are down there you can use your elbows to push your knees out and wiggle around. most people feel a good stretch doing that.
with ankle flexibility you can stand in front of a wall and do little pushes trying to get your knee to touch the wall. then move your foot back a bit away from the wall.
some people find that using a foam roller / lacross ball gives them more mobility than they get from stretching alone. e.g., rolling away knots under the feet, up the calves, up the front of the shins can often instantaneously improve ankle mobility. rolling around the hips can often improve squatting mobility.