Save money make busbars from aluminium .
Just make them a bit thicker a wider , easy to come by alu strip and a lot cheaper.
and you can dril holes en tap screw treath in them for the measuring wires .
Cheap crimp terminals generally
12-2 12-3 for example is three conductor cable with no indication of stranding and generally is house wiring. 12-3 WG is three insulated conductors with one bare ground.
Wire gage is not a range
the link above is useful: see photo
as shown multiple sizes can be wrapped in a single ‘cable’ for various purposes. Like audio-use cable that contains 9 (or 11?) different conductors, trailer cable that has 7 or 8 conductors and three or for different gages depending on color/function.
18-3 is generally tstat or alarm wire and not probably applicable to solar other than maybe monitoring devices.