How do the medics know it is tendinopathy. Have they done an MRI showing tears and other damage? If everything you have done so far has come up with zero then you are fixing the wrong place. Let me give you an example. For many years I suffered from lower back pain. I did all the right things with the exception of spinal blocks. No thanks. After twenty years I saw a myofascial therapist who immediately diagnosed overly tight ilio psoas muscles. He worked on me for 3 sessions and gave me exercises to do ( very simple ones too) and after 20 years of everything else under the sun aimed at the lower back itself, presto no more pain. My guess is your tendons are inflamed and angry because of the way you sit, or walk or sleep. Sitting is my first choice but I don't know you. Doctors and others in the medical profession are not trained to see much beyond what they learn in medical school and that isn't very much except pushing pills. Find a John Barnes fascial therapist or a Tom Myers Anatomy Trains practicioner or a Rolfer. With a 10 minute exam my bet is that they will come up with the root cause. Just for fun google can a tight psoas muscle cause gluteal pain ?