Hey, Subasaurus, great idea using the copper tube for terminals!!
That's probably the base for commercially made terminals anyway, they just "tin" it to keep it from turning green.
I too love the butane soldering irons. They work great and are completely portable.
I do love soldered connections, but you have to keep a couple of things in mind:
Connections subjected to high vibration, could potentially fracture because all the "give" in the wire was taken away with soldering. I personally have never seen an example of this, and don't believe it is as critical as some claim. Especially if the wire is supported with the vibrating component, well past the wicking point.
An additional mechanical connection is preferred, (and required by the NEC in most cases, even though not relevant to automotive systems) because given enough current and resistance, solder will melt.
Those ratchet crimpers do a great job up to a #8 wire. Especially the ones with the dies that actually stuff the open ends of the terminals down into the center of the wire like OEM.