Thank you for the question.  You have some decisions to make.  If you prune the entire shrub back close to the ground now, you will remove the wood that contains the flowering spring buds and you will have no flowers next spring.  You could decide to cut it back completely next spring after it blooms so that you can enjoy the flowers.  If the shrub is healthy, it will respond with strong new growth that will need to be periodically pruned for the health and form of the shrub.  

You point out that this shrub is an important plant in your landscape.  You may want to review the publications provided here before deciding that rejuvenation pruning is not an option. In rejuvenation pruning, 1/3 of the oldest branches along with diseased or dead branches are removed every year.  This results in a gradual improvement of shrub size and form without sacrificing its presence as an anchor plant.  This pruning would take place after spring blooming is finished.

Sometimes a shrub is so overgrown and branches so tangled that it is better to remove it and replace with a new plant suitable to the growing conditions.  

https://njaes.rutgers.edu/pubs/fs1221/ 

http://www.ext.colostate.edu/mg/gardennotes/616.html

http://extension.psu.edu/plants/green-industry/news/2012/hacking-whacking-dealing-with-overgrown-shrubs

Thank you for contacting Extension.