Thank you for your question, Christina.  I wish you had taken photos of the leaves when they were still green, and the OSU Plant Clinic could have examined them.  With the information we have, the most likely culprit is one of the fungal diseases described here:  https://pnwhandbooks.org/plantdisease/host-disease/prune-plum-prunus-spp-leaf-spots

Pruning isn't going to cure the problem, if it's a fungal infection, and defoliation is going to happen this fall anyway, since this is a deciduous tree.  But pruning now will open up the tree to increase airflow, and help reduce moisture buildup.  And, as the article says, removing leaves as they drop will eliminate places for the fungus to overwinter, only to re-appear in the spring.

You can spray any of the chemicals in the "at leaf fall" category, with any one with a "H" (homeowner) label.  In the spring, you can apply one of the "H" chemicals, per label directions.  Next year, if the problem persists, send leaf samples to the OSU Plant Clinic while they are still green.  https://bpp.oregonstate.edu/plant-clinic/submit-sample/how-submit-plant-materials

Hope this is helpful.  Good luck!