I never had much luck growing plums, and yours look lovely! I do hope you get a break from your heat wave, and some rain.
If those tall green plants are mugwort then they're certainly different from mine which are perhaps a foot tall with white spotted leaves and pink or blue flowers.
Maybe make some sleeves for your tree? Still be enough for the birds but then you can get them fully ripened
Nice to read a local (ish) blog. Interesting bit about cherries. We have a huge sweet cherry tree. Every few years there are enough for birds and us. Had a few this year, fully black when properly rip