Hi, 

Have you gotten the soil tested? The sulfur probably helped with the pH level of the soil, but without knowing what it is and how it should be adjusted, it may have not been enough or potentially too much (unlikely since it takes a good amount of time to adjust soil pH).

Typically Rhododendrons prefer fertile acidic but well draining soil, so the sandy soil and root competition could have been too much for it to become established. Potentially it was too dry for it?

The symptoms look typical or Phytophthora dieback though which is a root root fungus. Stressed plants tend to be effected mostly. You can refer to our diagnostic page for Rhododendrons and Azalea problems and there are tips for growing rhododendrons and also what to look out for in terms of signs or symptoms of stress. 

This rhodie may be a lost cause at this point and you could either try again or find a more suitable plant for the space. If you do want to plant another one, try amending the soil with organic matter like compost or LeafGro. 

We would be happy to help with suggestions if you are interested. 

Emily