A really nice garden, I must say!
I still don't think your Emerald Green arborvitae are suffering at all. The hydrangea aren't damaging the trees' health, but they are modifying the growth. Wherever the tree branches are shaded heavily, growth will be thin or lacking. You might have noticed that the trees' branches aren't nice and green where they are against the fence (and dark), either. This is normal. When branches and leaves don't receive enough light to make enough energy to support themselves, the tree shuts them down.
So it is not that you need to choose between the hydrangea or the Emerald Greens to grow, but you have to accept that wherever the tree branches are heavily shade, they will be thin or withering. You could trim out the other lower branches of the arborvitaes to match the bald spots caused by the hydrangea. This would give your garden a completely different look, both summer and especially winter, so you will need to decide what you want. If you just let things be as is, the problem (which is only aesthetic) will continue. Every garden is dynamic, and constantly evolves. No garden is static (unless it is plastic foliage and flowers). Another possibility, if you like hydrangea, you could replace the present ones with a dwarf variety that stays smaller in stature.
By the way, I grew up in Minnesota, and knew the wild arborvitae as White cedars. "Cedar" is a colloquial name we northerners have for arborvitae, even though they are not the true cedars that grow in warmer climates.