I think czar is know for handeling shade well. Don’t expect the same production/sweetnes as in more sun though. A lot of the wild small cooking plums also seem to do fine in shadier positions. I think their called damsons in english.
For your ally, i would also consider earlier ripening “normal” plums. If the shade is from a fixed object. It will shade plants a lot in the early spring and late fall. But might barely shade them mid summer when the sun’s higher in the sky. and thus fruits that riping during that time, still can get quite good.
I would still go for easier varieties in a spot thats shady during fall. Lots of shade during fruit bud developing times mixed with a hard to get dbearing variety seems like a bad match to me. Although if had no experiance with that, so it’s just speculation.
you could use the shadowcalculator i mentioned and explained in this topic
It could show you when in the year you have least shade, and how many houres of sun you get at that point.