An entire hub ready to go i the better idea. imo
We had a heavy boat two axle trailer, that got immersed quite a bit. I was fanatical about greasing them. I checked them every time I stopped when traveling distances. They used cheap Chinese bearings, which in the eighties and nineties much worse than today. Mine became loose from the destruction of the bearings. When I got hom I took old bearings and got the correct size in Timken, then American made. Biggest problem not having a press was getting seals out with screwdriver, chisel, punch etc and hammer and driving the bearing races out with large socket and hammer. That took some doing. I put Lyman reloading cartridge case grease, on the new races and drove them in carefully and much easier than taking the old ones out.
It never occurred to me to carry a complete spare assembly. Some places they go out you might be a while, like above Thunder Bay before help gets there. I might have gotten away with no wheel on that axle but I drove slow and filled the hub every little bit, til we got home. It was messed up alread so no more real damage unless it was to seize.