You’re on the right track - it really doesn’t pay to reload for .458wm. Or maybe it’s better said as, it really doesn’t cost much to not reload for .458. Out of the DG cartridges, factory ammo for the .458wm is really quite affordable, and quite readily available. I like to reload for mine, simply to give me access to lighter loads.
Alternatively, as you found this rifle, it’s not so difficult to find great deals on 458wm dies, when one-timers get out of the cartridge after their trip.
Good for the salesman who convinced the original buyer he needed a .458wm to hunt pigs! A slick met a sucker that day!!