I don't like to solder too many connections. The heat weakens the copper and the flux can sometimes lead to corrosion. I've found, over the course of decades, a good, solid crimp lasts longer than soldered lug connections, especially if there will be mechanical stress or vibration. I will however solder wire to wire to make a T joint, but that will be wrapped within the harness so all of the other wires will be supporting it, and there will be no stress from disconnecting/re-connecting a device etc.
I've also never had an issue with lighting or ignition needing a relay as long as the wiring was in good condition. If the wiring is in bad condition, I like to fix/replace that.