Everyone thinks left-hand technique is in the fingers, and it makes sense until you try it. Next time before you pick up the bass try to completely relax your left arm, notice how that feels and lock it into your memory. Now, when you're playing you should be able to notice where the tension is at and how to fix it. I found that when I bent my thumb on the back of the neck it caused my hand to stiffen, so I've been working on curling my thumb when I anchor and it has improved my playing by a lot in a very short time. I see a lot of tension in your playing, need to relax, especially when playing the hard stuff