And it sounds like you are taking pride in the history of the rifle. A decision is needed. Keep it completely "original" or as suggested, get an adjustable gas plug? If you keep it GI spec, only use GI spec ammo for best results.
The thing with Garands is that the design was optimised to run on the GI ammo. Commercial sporting ammo is unsuitable. It isn't a question of the strength of the Garand action, or the ultimate pressure of the ammo, as muc as it is one of the gas port pressure, and pressure curve of the ammo.
Hunting ammo that is perfectly fine in a bolt action, pump, or commercial semi auto usually works the Garand action "violently". Over time (an no one can say exactly how much) continued use of sporting ammo usualy results in a sprung op rod. And maybe battering to other parts as well.
A few rounds seldom does damage, but it is not a good idea in the long run, if you value your Garand. Stick with GI ball (M2) or a handloaded equivalent and your Garand will run fine for a long, long time.
All else being equal (and it almost never is) bigger bullets tend to work better.