I forgot to mention that I have also been using a water-soluble general-use fertilizer once every two weeks or so.

Unless there is a soil test that indicates that something is missing, stop with the fertilizer. You can create issues with excess fertilizer, both in the garden and in the surface water and groundwater where the excess ends up. As an indication of how much fertilizer this plant needs, in my nutrient poor acid sandy loam soil, without any fertilizer ever, mine has done fine, blooming profusely and taking over as much territory as I will allow it. And regardless, you want to stop fertilizing in August so the plant stops putting out new growth for the year that might not have time to harden off before winter.

Do you check to see how moist the soil is before watering? And how long do you water for or how much water do you give it?

I would look at the underside of the leaves to see if there are insect pests, but other than quitting with the fertilizer and checking to see that you aren’t over- or under-watering, I would chalk it up to being late in the season, and as a new plant with less developed roots that gets a lot of sun, the leaves are a bit tired. Typically, this is planted where it gets some shade in the afternoon when sun and heat are particularly intense.

I looked under the leaves and there do not appear to be any insect infestation.

Would you say the same for these as well? They are a newer addition and are of the endless summer variety.

Again, no insects on the underside of the leaves

Yup similar late summer, new planting, and perhaps more sun than preferred. Time will tell on the last. you should be aware that Endless Summer typically is root hardy in zone 5b, but you will not see spring flowers most years unless you winter protect them since either cold winter temperatures or late frosts will kill most if not all of the flower buds. Late summer flowers will be somewhat sparse. Do a search for winter protection on the Hydrangea forum to get some ideas on what to do and success rates.

I agree. Either an insect/pest or sunlight. If it is sunlight, only leaves in direct contact with the sun will be affected. I also see some minor leaf spot in the second set of pics (water only the soil and not the leaves, if you can). These leaves will soon fall off so not a big problem. Take pictures of the same leaves every two or three days and see if you detect more leaf damage (by insects or the sun) or not.