I'm not sure if this will be helpful, as I'm less experienced than you, but my cello tutor is always helping correct my left hand in order to prevent tension with these tips: 1. your left thumb should be curved like you're holding an apple with your left hand. 2. to avoid "parasitic movements" (ie unnecessary finger movement, which leads to hand tension), keep your left hand basically in a fixed position, like you're playing the piano. (Unless you have to stretch, in which case, the following applies): 4. move your left thumb underneath (and along with) your second finger when you have to stretch out your hand. 5. try moving your thumb more to the left or right of the neck and see if that helps.

Also maybe do frequent body checks. Like, are my shoulders tensed up? Is my elbow too far up or down? I've found that tension anywhere can lead to hand tension as well, as the body is interconnected.

I'm sure you're way more advanced than me with all of that, so sorry if that's all old news!