I would suggest to replace grease by oil.
One difference is that grease stays where it was when you stopped the trailer.
I suppose that when you dismantled the hub you found the grease stayng all arond the external sleeve of the bearing. When it is like that, it starts to become an effective lube when the roller became hot enough to make the grease melt. Too late !
With oil, the lube remains down, and keeps its action everywhere it is, so it protects the bearing sleeves and rollers.
Plus : with oil, the acceptable load can be x3.
To replace grease by oil, you must have a leak-free system like O-ring, L-ring, ... contacting an added sleeve, ie a very thin corrosion-free surface, like Stainless steel, bronze, brass (easy to find) ... added/glued with epoxy onto the wheel shaft.
Then to make the hub dustcaps leak-free too bu glueing them with polyurethane sealant (Sikaflex)
You will spend some money to have the special sleeve machined (one time I made 1mm thin wall ones, so I was able to use the genuine oil-retainer) - or you could buy ready-to-use sleeves made of porous bronze, with larger external diameter, and buy the corresponding size oil-retainer.
But you will never have to change again your bearings !
Should you have any problem to do that because of hard-to-find dimensions, I have all the equipment, lathe ...