One thing to think about is that 6 mg of Prednisone is a "fixed dose" but that isn't how the body regulates cortisol. The body is designed to get bursts of cortisol during stressful situations. I would think lifting weights is a type of stress. Your cortisol level should increase when weight lifting but Prednisone suppresses your adrenal function. You won't get the cortisol response to weight lifting that you should get.

The pain you feel when weight lifting could stem from the body's need for more cortisol. Unfortunately, increasing the dose of Prednisone to facilitate weight lifting isn't appropriate.

It might be a mistake to push through the pain when you do weight lifting. Maybe you could reduce the amount of weight you lift.

Another thing that could be happening is something called insulin resistance that is caused by Prednisone. With insulin resistance, the energy from glucose isn't delivered to muscle cells during weight lifting. This can also explain some of the pain you have during weight lifting. Muscle recovery times after weight lifting will also be prolonged because of adrenal suppression. Adequate cortisol function is essential for a normal recovery of muscles that are used for exercise.

I think exercise is vital for PMR. Unfortunately, the prednisone we take to treat PMR makes exercise very challenging. I like the advice you give to start slowly and gradually increase the amount of exercise over time. It doesn't have to be weight lifting on dry land. Water resistance during aquacise can also be very beneficial.