Good afternoon, and thank you for your question. Based on your images and description, it appears you have a couple of things going on at once. First, your soil pH is perfect for rhododendrons — it was smart of you to start with a soil test.

The irregular holes punched through the middle of the leaves (rather than along the edges) are typical of a heavy black vine weevil population. The adults feed at night, which makes them difficult to catch in the act. The larvae overwinter in the soil feeding on the roots, and by late spring the root damage can be significant. Weevils may also have been responsible for the failure of the previous plants in that area.

You also appear to have rhododendron leaf spot. The lesions look quite different from the weevil damage — this is likely Pestalotiopsis or Phytophthora leaf blight, not powdery mildew. Isolated leaf spotting like this commonly appears on plants that are already stressed by weevil feeding, harsh winter conditions, or root damage.

Your most pressing issue is the weevils. I have had good results with soilborne pests using beneficial nematodes — you simply mix them in water and apply the solution around the base of the plant. Soil temperatures need to be at least 50°F, and evening application is best. The biggest risk to nematodes is allowing them to dry out before they work their way into the soil, so keep them out of the sun when applying. The recommended species for weevil control are Steinernema kraussei or Heterorhabditis bacteriophora. From late May through June, adults will emerge and begin actively feeding, at which point you may want to consider applying spinosad or pyrethrin — be sure to read and follow all label directions.

For the leaf spot, remove and discard any damaged leaves (do not compost them). Apply a fungicide to protect the remaining foliage — I have had good results with copper-based products, though you will need to reapply after rain events. Be sure to read and follow the label directions. If possible, improve airflow in and around the plant by thinning any overcrowded growth.

I have included a few links below that describe these problems and support my recommendations.

Good luck and happy growing!

https://extension.psu.edu/black-vine-weevil

https://extension.umd.edu/resource/fungal-leaf-spots-shrubs/

https://ohioline.osu.edu/factsheet/ENT-49