Muscle and joint pain is a symptom of adrenal insufficiency. Hydrocortisone is the preferred treatment but Prednisone will also work.

Myositis would account for the symptoms you are describing.
https://my.clevelandclinic.org/health/diseases/24170-myositis
Muscle weakness in the area of the throat is consistent with myositis. An elevated CK level is also consistent.

Myositis and pulmonary complications are part of the clinical picture too
https://pubmed.ncbi.nlm.nih.gov/20692542/
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The following is a good overview of polymyositis and it also compares PMR and polymyositis.
https://www.webmd.com/arthritis/polymyositis
I'm not a doctor but I think you should reconsider steroid treatment. It seems like you will need steroids sooner or later.

"Sometimes, it seems like I cannot possibly hold my head up any more on its own."

You need a thorough evaluation sooner rather than later! Myositis can cause this symptom too but I wouldn't want to speculate about this one.