That amazes me that DW suggests they ripen in Sept in CA- that is when they've ripened here the last couple of years I've harvested them and I'm over a month later than you with most varieties. Z6 southern NY.
I can see how they could be difficult to determine ripeness for because they ripen while still firm and crunchy. You bring them inside to further ripen if you want a traditional plum texture but I'm in love with the crunch.
They've become one of my favorite J. plums. As I've stated before, I call them that because that is what they seem to me. "Where's the beef"- I mean cot. Every type of pluot I've tasted only manifests plum genes as far as I can tell. Who cares? A great fruit is a great fruit by any name. I think Shakespeare said something like that about roses.
Anyway, if I was you I wouldn't try any until at lease mid-July.