My 14 year old English Springer Spaniel started taking pred for IMHA. She bottomed out pretty quick and I almost lost her. Vet diagnosed her quickly and we got her back. Found the lowest dose that kept her going. She did well for 2 more years. Eventually long term pred will take it's toll. The immune system figures out you've been screwing with it and will ramp up.
I could not ask for any more from her, and she went on ahead to wait for me at the bridge just about a year ago. Pred is one of those drugs that will help but hurt in other ways.
(I have just welcomed a new furry soul into the house.) Good luck.