Do you know if your rhododendron cultivar is supposed to grow to 8-10'? There are many dwarf rhodies that will stay very compact - 3-4' or less. Unless you can confirm what kind you have, I wouldn't be messing about trying to correct some sort of root defect that may or may not exist,

btw, rhodies are by nature extremely shallowly rooted.

No, this was definitely a large plant. It was gigantic in it's pot. As time passed, the size of the flowers and leaves started to decrease.

I have the cultivated and also the wild. it was so root bound it was solid white roots when I took it out of the pot. Back then I didn't know what to do. It's very frustrating because it was the biggest plant I ever bot and a good price.

(I just bot what appears to be a dwarf - it's a completely different type of plant. I have a row of privet hedges that are over 50 years old and this last year, two of them died - the very 1st deaths. I plane to "pepper" that row with a bunch of different plants. Also have a nice azelia that matches the color of my current Rhodi's. Also on the lookout for the Rose of Sharon.)

I consider a dense root ball perfectly normal for rhodys. Chopping away roots will not help growth.

Is there some barrier around or beneath the root area?

Have you been fertilizing this plant at all?

Have you considered top-dressing with compost several times a year?

It is root bound. When removed from pot, so root bound you could not see dirt was white with solid roots. I knew I should have researched it then, but just put it in the ground. I have fertilized it every few years. It is planted it regular soil. No composting. Nourishing it has no appreciable effect.

No nutrients = no growth

Pop it out of the ground. Take a sharp knife and slice from top to bottom through the EDGE of the roots. Replant, making sure it is NOT deeper than it was to start. Take a garden fork and loosen the soil around the plant The roots are pot bound and keep going in a circle. .

LEAVE IT ALONE, except for water and mulch.

I had a similar problem once and killed the plant trying to unknot the roots. So be careful, certain varieties of rhododendron can have brittle roots that easily snap.