100gr of even Pyro P and a 300gr GoldDot/DeepCurl with a Harvester black Crushrib should be very similar to the 300gr Fushions...and totally safe in nearly any muzzleloader.

You shouldnt see any sign of too much pressure with that load IMO. Ive used much hotter charges with 300gr bullets and the black Crushrib without any signs of failure.

What gun are you using? Different guns react differently and early signs might be more obvious with some of them.