[Jim in Wisconsin]

In my working career, I maintained a fleet of electric lift trucks so I dealt with d.c. connections a lot. A good crimp is a better connection than a solder joint if the lug is desiged for that. If a wire is a loose fit in a lug and the gap is filled with solder, that would not be considered a good connection, and letting the solder wick up into the wire is also not good.
If these bullet connectors aren't designed to be crimped, then solder is the only alternative. Copper conducts electricity better than lead (or tin or whatever modern solder is made of).
If the wire was a loose fit in the lug I needed to use, I would sometimes strip the wire twice as long and double it over to make a tight fit and a better crimp.