Mophead Hydrangeas too big
From the photo and your description, it looks like you have big-leaved hydrangeas that flower on new wood. There are some newer cultivars that flower on new and old wood, but since yours are 20 years old, yours probably only flowers from new wood, i.e. the present year's growth. That being said, your shrubs might have outgrown their area, but they don't look too overgrown as they can get quite large and are often used as a specimen planting.
You can prune out dead wood in the spring but this will result in additional shoots coming up and filling in the area. You can divide your hydrangeas this coming spring, and this will reduce their size, i.e. divide and move 1/2 to another area.
Here is an extension publication that explains more about pruning and types of hydrangeas: https://extension.umd.edu/hgic/topics/guide-pruning-hydrangeas