I think a brix at 11-12 is considered mildly sweet. I ate my not fully ripe Satsuma, Satsuma tasted definitely sweeter but I could not get juice to measure its Brix yet.
Last year, I stared picking Shiro on Aug 14. So, about the same time.
We did not have lot of rain during those picking weeks. To me, Shiro is mildly sweet when compare to other plums I have eaten.