Your van interior artistic work is amazing. My wife is the artistic side of our family and it is pretty obvious.
I help people locally with van electrical and I have a slightly different take on "wire making".
The bulk of the wire runs from the "main system" to the "appliances" can be done with 10 awg and some 14 awg for things like lights. If you are into DIY wiring, save this portion for yourself.
There is no great joy that comes from cutting and crimping wires that are heavier duty that 10 awg. It goes from "fast and easy" to "much more difficult and time consuming".
What I do when building a new design is to mock up everything in 10 awg and and get it all functional.
Anything that needs to be heavier duty, I just have an outside supplier make those wires from my mock ups. There is a reason that these heavy guage wire making businesses exist. (economy of scale).
They have invested in the correct heavy duty tooling, have calibration procedures to make sure that the crimps are coming out right, and buy wire / lugs in volumes that allows them to sell a very high quality product to you for less than most people can purchase the parts.
Simply from a time perspective, I can order 10 - 20 each, 2 awg or 2/0 wire exactly the way that I want it on line, and have it delivered in 3 days.
I am not a fan of ebay but there are some very good, high end wire making suppliers on there. Marine cable is nice but a lot of it is not rated for the lower temperatures ranges because - boats aren't normally on the water when it is ice.