Adding an antifoggant will not fix the apparent contrast problem AFAIK. It will give you white on gray instead of gray on darker gray.
Benzotriazole probably would be best. I would start at 1 mg/l and you could need to go up to 10 mg/l. I would test the paper after each incremental addition, so at 1 mg, 2 mg, 4 mg etc, and at some point the fog will vanish. Speed will also decrease. With this paper, you may even need to go higher.
Sometimes you are chasing your tail though. You add BTAZ and lose fog and speed so much you have to develop more and that gives fog, and etc....