I'm not a vegan either but I eat a lot less red meat than I used to and eat more poultry. I wished I could eat fish but I get sick when I try. I do think diet plays a part in getting the PMR into remission sooner and getting off of prednisone. I had the same issue with mostly a desk job over the years and moving around does help. Here's some suggestions on foods that can help for PMR - https://www.arthritis-health.com/blog/diet-and-supplements-polymyalgia-rheumatica-pmr. Reducing fried foods is a great start along with portion control and eating more veggies.

I agree that diet can play a significant role. I cut red meat, processed/junk foods, and sugar from my diet. I found that cutting sugar made the biggest difference in keeping my PMR symptoms under better control as I taper my prednisone dose. I'm also working at lowering my salt intake to prevent my blood pressure from getting too high, but I find that more difficult to do because so many foods taste like nothing without salt, plus there's so much added sodium in the foods we buy. My biggest stumbling block, though, is carbs -- I love rice and pasta (gluten-free) and eat a lot of both, even though I know I should be cutting back on those too.