I just started picking lower leaves off Redbor and Cavalo Nero. Some years I have picked them hard, leaving just a few small leaves at the top of plants by December, but this does make for slower regrowth. It depends when you most want leaves. Cavalo Nero also makes sideshooting stems with smaller leaves.
Also this year I am growing True Siberian (Real Seeds) and Westland Winter kales, so they will be interesting to compare against each other. And there are a few plants of tree cabbage…