Awesome job with finding that relay & building those connectors!!!
Fondly reminds me of my teenage years hanging out at Sheldon's Hobbies getting latest for RC cars.
Soldering is really only way to join wires to terminals reliably. Contact surface area is immensely larger than crimping. Can handle much more current and is one of main reasons behind that requirement in aerospace and military applications.
Crimping without soldering is used for cheap and fast production, but has serious drawbacks in durability. OEMs do it for fast production lines, but it comes back and bites them due to corrosion and eventual overload. Kawasaki has numerous faults in their harnesses on many bike-models where multiple common ground-wires are joined via crimped 4-way connectors. These corrode over time due to moisture and arcing from limited contact surface-area fries joint and it fails.
Toyota too has numerous issues with multiple-grounds joining (blue connector in left foot-well). After while, plastic connector housing melts due to heat from high-current flowing through restricted contact-area. It affects entire product-line, from lowly Corollas all way to top-of-line Lexus.
I used to make replacement battery & headlight harnesses for Porsches. Just replacing harness alone increases light-output by 25-50%. Every single wire is pre-tinned entire length for corrosion-resistance, end is crimped, soldered and heat-shrink wrapped according to
NASA manual.
Battery harness with mil-spec zinc battery clamps (better conductivity, non-corrosive)
Headlight harness
And I warranty them for 50-years. It's been 25-years now and not single one has come back. People will remove my harness from their cars when selling and move them to their new cars. They occasionally pop up on eBay being sold for higher than my original pricing!