Rather than continuing with the Buckthorn, I'm turning to a crabapple setup. I feel that I pushed intensely dark values too quickly and also had a lot of trouble with layer lifting.
So here I am trying a larger image (6x8") and working with subtle values. And most importantly, I am working hard to hide brushstrokes. There is some blotchiness but I'm learning to stay out of trouble. Patience and attentiveness are key.