This kind of leaf spot is very common on both oakleaf and bigleaf hydrangeas, and while it might spread into the landscape, it's likely not much higher of a risk than having spores arrive in the garden from other sources (they are windborne and/or rain-splashed around). It's uncommon to see a mature hydrangea shrub without some degree of leaf spot, honestly, since these diseases are so widespread. Both fungal and bacterial leaf spotting pathogens occur in hydrangea, and while we can't tell exactly which one this is, they rarely require treatment with a fungicide. (Besides, fungicides don't always work to prevent disease, and might risk harm to pollinators or other organisms. No fungicide is curative; they only help protect healthy growth from infection.)

Fortunately, since they don't cause long-term plant harm and are only an eyesore, plants with leaf spot should be just fine. You could individually snip off the worst of the infected leaves, but that might deprive a plant (especially a young specimen) of foliage that is otherwise perfectly functional and still fueling growth via photosynthesis in the areas of the leaf that are unaffected. When its leaves shed this coming autumn, you can rake them up and dispose of them to help reduce the spore presence for next year, though it's not a foolproof measure since they could always blow-in from another source at any point in the future. Planting in a location with good air circulation (such as not too close to a wall or solid fence) can help reduce the risk of leaf diseases since the airflow helps foliage dry off faster after rain/dew/irrigation, as wet leaf surfaces tend to be easier for pathogens to infect.

Wilted branches are hard to diagnose but could be a symptom of over-watering, under-watering, twisted-broken stems, or an isolated infection or stem-boring insect damage. In either case, isolated branches can be pruned off. When an entire plant wilts and doesn't revive, we'd guess over-watering was the culprit (possibly under-watering that took too long to remedy), but it's hard to say.

Miri

Thank you Sherrell, and you're welcome!