From this description, it is very clear that whatever you did to ensure drainage was probably not effective. Can you describe how you tried to provide drainage, and even better would be to take some photos of the base of the plants and the surrounding area.

Compost does not improve drainage. Compost in fact does the opposite because it is made up of many small particles that hold onto water. In commercial farming, compost is never supposed to be more than about 10% of the soil composition. In a rhododendron application, you want your compost on the top of the soil, not so much in the soil.

Thanks for all the help. I may also have overwatered in the beginning but as soon as I saw changes I cut back to as little as possible. I spoke to someone at the state extension office who asked me for photos, which she said confirmed the root rot. I thought about driving the plant down to the local office which is not too far away and get a confirmation. Here are some photos of the plant

Overwatering, planting too deeply and poor drainage could all be contributors. The first two are rather easily corrected.....the last is not. I would address drainage issues thoroughly before considering any replanting. And always plant rhodies high.....they have a very shallow root system and a portion of the root ball should be at or slightly above grade.

Your first two photos seem more like this:

https://pnwhandbooks.org/plantdisease/host-disease/rhododendron-phytophthora-blight-dieback

Note in particular the opening paragraph which says

Many different Phytophthora species, fungal-like microorganisms, can cause blights and dieback of Rhododendron alone without necessarily producing a root rot phase. Phytophthora syringae, is common during the cool winter months. P. plurivora is common during warmer months and is most active in summer. Phytophthora cactorum, P. citricola, P. parasitica, and P. heveae also can produce leaf spots and dieback and are also active in warm summer weather. Other species active in the PNW include P. gonapodyides and P. hibernalis. Also, P. ramorum is yet another species that can cause similar symptoms but is a quarantined pest. Not only can one find a different set of species at different times of the year, but different sets of species can also be found from nursery-to-nursery.

@annecorea Can you take a photo with a wider scale to show the overall area these are planted in? Does this part of the garden puddle with water during rain?

I would build a raised mound in this area that is a minimum of 18 inches high and probably two feet would be better.. Plant the rhodies in that raised mound, and if the water is puddling in that area of the garden I would not even let the base of the roots go to ground level.

What kind of soil mix did you plant them in? Probably Gardengal48 will have better ideas on what soil mix to use on the raised mound.