Just saw your post. I have only flavor supreme and flavor king grafted on a peach tree (added small Santa Rosa branches this year and had a Santa Rosa plum tree 50 yards away) and flavor king has absolutely loaded like grapes. Flavor supreme also fruits well but not as crazy as FK. Just FYI.
Btw, I am in mid Atlantic area. It’s said FS was difficult to fruit for many people in east coast but not my experience.
Another thing: I bought the Santa Rosa last year from Lowe’s because I was told that FS and FK had difficulty pollinating each other. But my 1 year branches of FS and FK fruited profusely last year. I also grafted FS and FK on Santa Rosa last year and this year FK fruits like grapes and FS also fruits very well on that tree. After thinning I have about 20 fruits on that small tree including 4 Santa Rosa.
So the mystery of plums/pluots pollination remains mystery at least for me, but in a good way.