Apriums sound very tasty. My mom has a red plum tree, but the fruit is not at all sweet. Maybe an aprium made from that kind of plum would be more tart, but still tasty.
Sour as they are, those red plums make a wonderful jam. It's tart too, but the flavor of the plums really comes through in the jam. It's sweet enough that you can enjoy the plum flavor without being overwhelmed by the tartness. I'm not sure what species that plum tree is, but it's been there a long time. I don't think they planted it, either. I think it came up volunteer.