Good luck with Actemra and I hope it works for you. It will take some time for Actemra to work. My rheumatologist said it would take about 3 months for the effects to be felt. Perhaps it was the power of suggestion but I tapered by 1 mg per month for the first 3 months. After 3 months I tapered by 1 mg per week. This was starting from a 10 mg dose of prednisone.

This doesn't mean you will be able to taper off prednisone in 3 months. I needed to stop my taper at 3 mg for a period of 6 months because of adrenal insufficiency.

Starting prednisone for autoimmune problems is an easy thing to do. Everyone likes the fast pain relief. Unfortunately, the longer you take prednisone, the more adrenal suppression there is. It takes time for adrenal function to return and that is why people are told to taper off prednisone slowly in the first place.

Sometimes the symptoms of secondary adrenal insufficiency and a low cortisol level can mimic PMR symptoms. This becomes more and more apparent when you get to lower doses of prednisone.
https://thesolutioniv.com/blog/low-cortisol-levels-causes-effects/
Cortisol is the substance that "regulates" inflammation. I think when a person's cortisol level gets low, they may feel inflammatory "flares" which may or may not be PMR/GCA related.

If Actemra allows you to get to something less than 5 mg of prednisone, that will be low enough to give your adrenals the chance to begin the recovery process. Actemra doesn't have the adrenal suppression effects that prednisone does. In general, that is the purpose of all "steroid sparing" medications.