I have RA/AxSpA and severe muscle weakness off steroids which led to atrophy recently. In addition to Medrol, Methotrexate and IV Cosentyx (for the RA/AxSpa), my doctor recommended swimming before this last flare, and I found it helped tremendously with joint pain and helped to build strength. I was often tired afterwards, but I had no severe muscle pain, and I built up the time in 5 minute increments slowly over 6 months. I also go to physical therapy twice a week. I am not currently able to swim as my arm and leg muscles are weak. but I am continuing PT pending testing results for an additional neuromuscular condition.

The Myositis Association has a list of medications that are currently being used for Myositis. My infusion nurse mentioned she has a patient with Myositis who has done very well on IVIG (Intravenous Immunoglobin) treatment, which is mentioned in the link:
https://www.myositis.org/about-myositis/treatment-disease-management/medications/
Here's a link to suggested Exercise and Physical Therapy for Myositis patients:
https://www.myositis.org/about-myositis/treatment-disease-management/exercise-physical-therapy/