First, welcome to the world of "It doesn't make sense". Being a doctor one would think that the mysteries of PMR could be better understood by a medically trained professional. As you realize PMR is not understood by the medical professionals tasked with treating it, however the treatment has become much more "standardized" in the seven years PMR has been my constant companion.

My understanding of the standard school of thought now is after initial treatment to gain control of the pain that the taper to 20mg should be swift. If more than 20mg of prednisone is needed to control pain, then likely PMR is not the only player being treated. Once 20 mg is reached then real tapering begins. Tapering to 10 mg is often done at 2.5mg rates per month, then starting at 10 it is most often done at 1mg/month as long as pain does not return.

You are in the below 20 taper heading to 10 and have hit the pain wall at 15. From what you told us, you are pain controlled at 20, but for now cannot get below 15. With the addition of biologics I am not sure how this changes the taper expectations, but I am sure it does. Discuss this with your Rheumy and see where he lands. There is no single correct answer, but at some point I would suggest you request additional testing to see if a non-ANCA vasculitis is also in the picture. Remember, PMR is a default Dx and to date there is no test that can be ordered to confirm the Dx. The best they can do is test to eliminate other causes. I would not want to be a Rheumy.

Long story, but I requested vasculitis testing in 2023 and they only ran ANCA blood work that was negative. Three years later asymptomatic GCA Vasculitis was Dx and damage to my aorta is real. I could successfully taper to 0 but the inflammatory markers would begin to go back up with a vengeance almost immediately. This cycle repeated itself with every taper.