I owned and worked on my own airplane for 15 years under the direct supervision of a A&P and Inspector authorized designated engineer.
The main reason they don't use solder connections on factory airplanes is weight and wire gauge size as well as the airplane flying in all kinds of weather with massive air temp changes and vibration
I worked for the local phone company for over 33 yrs, we solder every permanent connections in the equipment offices and on all the inter connection wiring on most of the wires. If you solder and heat shrink the connections properly and make sure they aren't moving around a bunch you will be fine, as I'm sure Gus already knows that