I struggle with this too. But mainly if I try to get my fingers parallel to the fretboard with my left hand. It just feels beyond the capabilities of my hand. Not something that I can just work on. Especially true playing the E and A in the highest positions.
I know you’ve watched tons of videos, so probably have seen these, but the most comfortable position for me is the natural position, demonstrated in this video.
However, that doesn’t give me parallel to fretboard fingers without contorting my arm and wrist in a highly uncomfortable way.
The other thing that helped me was knowing that my thumb is just there as guidance, not as pressure. Ari talks about that in this video:
But as you say, I’ve combined these together and still really struggle with this. Be great to see what you learn from a private lesson on this. I’ve been wanting to do one for awhile, but lockdown has not allowed that.