Comparing the current crop of cheap Chinese bearings to the automotive bearings used for many decades is a fools game. Pumping excess grease in is nonsense.
I would never put any trailer, (new or used) in service before completely cleaning, inspecting and servicing the hubs and brakes. Better yet, toss the cheap bearings and replace with Timkens.
In PA, the brakes are state inspected once a year by pulling the drum. All that extra grease would make quite a mess. Following the state inspection the drum will need to be pulled again to correct the problems caused by the inspection, like wrong grease, dirty grease, poor bearing adjustment and contaminated brake shoes. Yes, I have been there.