I'd start thinning your rhody now. Take out all the little branches right down to the main trunk. Any branches that grow towards the center of the shrub. Any branches that are unhealthy. Branches that are redundant. This fall, head your shrub back by cutting the tips down to a bud that faces the outside of the shrub. Make the height 10'. Use alcohol on your pruners to clean before and after.
The problem for your shrub is that it is right next to the concrete foundation. Rhododendrons love acid soil. Concrete raises the pH and makes some chemicals in the fertilizer unavailable. I'd get a soil test to find the actual pH and then add sulfur. Fertilize now (unless you've already done so) with a fertilizer made for acid loving plants. Don't fertilize again until spring.
You also have the Japanese Rhododendron beetle. You can see the little nibbles on the edges of the leaves. That isn't the problem. The problem is in the roots near the bottom of the trunk where these beetles lay their eggs. The larva eat at the roots and reduce the vigor of your plant. Because the pH is off your rhody is more susceptible to these common insects.
Adding an article on this pest: Japanese Beetle. Please don't use the chemical pesticides. It really is not necessary. I am assuming there is a lawn just out of the pictures you sent. I'd try to talk with the previous owners about what they have done in terms of fertilizer and pesticide. Rhodys this large make beautiful LARGE bonsai looking small trees. Also be aware that plants don't live forever. This guy looks like he's almost reached his limit. Another question to ask the former owners. Do they remember planting this shrub?
Keep the pruning sessions spread out. Don't remove more than a quarter of biomass every few months.