There are several theories about who are most at risk of PMR.
https://www.arthritis-health.com/types/polymyalgia-rheumatica/risk-factors-and-possible-causes-polymyalgia-rheumatica#:~:text=Several%20potential%20causes%20for%20PMR,giant%20cell%20arteritis%20occur%20together.
I definitely think Covid and/or the vaccine are possible triggers.

I had some success with Methotrexate (MTX) when my rheumatologist wanted me to try it. I would say MTX allowed me to decrease my prednisone. Unfortunately my liver enzymes increased as my prednisone dose decreased. Overall, MTX just made me sick. Don't let me discourage you. I firmly believe you never know what treatmen might help you unless you try it to see.

Sweating and hot flushes happen to many people. It is likely a side effect from prednisone.

For your Dad to have PMR for 20 years seems like a long time. PMR is supposed to go away after a few years. There is a "proven genetic link" to many types of inflammatory arthritis. Those disorders never completely go away and tend to relapse and become chronic conditions.

I was diagnosed with inflammatory arthritis at the age of 32. When I turned 52 there was nothing that prevented me from having PMR in addition to inflammatory arthritis. Your diagnosis is what a doctor says it is. However, one autoimmune disorder is very difficult to distinguish from another autoimmune disorder. It is also difficult to know what treatment will work.

I do know that the longer I was on prednisone, the more fatigue I felt. This was probably caused by chronic inflammation and/or a prednisone side effect that caused adrenal insufficiency. Basically, the longer I stayed on daily prednisone, (13 years), the worse I felt. I'm off prednisone now and I feel infinitely better.