I was having episodes of temporary vision loss, so I started on high dose prednisone before the GCA was confirmed. I spent 3 days in the hospital and got an infusion of high dose methylprednisolone each day, and then I took 60 mg of prednisone per day for 6 weeks. They did the biopsy the second day I was in the hospital, and I got the positive results a couple of days after leaving the hospital. I started taking weekly Actemra injections about a month after the diagnosis. After the 6 weeks at 60 mg of prednisone, I tapered 5 mg every 2 weeks. So I stayed at or above 40 mg for roughly 14 weeks. I had a big problem with insomnia most of that time, but it improved quite a bit as I got close to 40.

I think that for you a negative biopsy will not rule out GCA. I found this information on Google:

"A temporal artery biopsy can still be considered valid even if you've been taking prednisone, but it's important to perform the biopsy within a few weeks of starting steroid treatment as the diagnostic accuracy can decrease significantly if you've been on steroids for a prolonged period; ideally, the biopsy should be done within 2 weeks of starting prednisone for suspected giant cell arteritis (GCA).
Key points to remember:
Early biopsy is key:
Due to the potential for serious complications from GCA, most clinicians recommend starting prednisone immediately upon suspicion of the disease and performing a temporal artery biopsy within a short timeframe, even if it means doing the biopsy while on steroids.
Decreased accuracy with prolonged steroid use:
The longer you've been on prednisone, the less accurate the biopsy may be as the medication can suppress inflammation in the artery, potentially leading to a false negative result. "

There is also the possibility that the biopsy just happens to miss an inflamed area of the artery. Here is another quote from Google:

"It is possible to have giant cell arteritis (GCA) even if your temporal artery biopsy is negative, as a negative biopsy does not definitively rule out the diagnosis due to the possibility of "skip lesions" where inflammation only affects certain parts of the temporal artery, potentially leading to a false negative result; if clinical suspicion is high, further investigation or treatment may be necessary despite a negative biopsy. "

I hope you get a clear result. Your case is a lot more challenging to diagnose than mine, because I was having the severe vision problems plus a lot of other GCA symptoms, and also I was not taking prednisone before I got diagnosed.

There is an interesting thread in this forum https://connect.mayoclinic.org/discussion/new-dx-of-pmr/ where @maye discusses getting off of prednisone completely in only 3 months by taking Actemra and tapering prednisone rapidly.