@lcherepkai So for slugs, you have a very short piece with a very large diameter. the higher specific resistivity of the other metals your are trying has little effect.
However, I think in speaker wires which are much longer and smaller in diameter the total resistance will be measurable and I think you’ll see significant degradation from the higher resistance of these materials, especially on bass and dynamic sections. So start with the least expensive.
As far as the original post of solid copper, there is obviously not a conductivity issue and people have been known to conclude that solid cores sound better than stranded so I think your plan sounds very interesting.
I also support your plan to eliminate connectors. connectors can be significant sources of bad connections, especially inexpensive bananas or any non-crimped/or soldered connectors. I use crimped and soldered naked bfa style connectors.
Best of luck,
Jerry