Are you talking about a type of wire, if so I would get a #4 thhn to pull the whole way, it has a slippery covering that you don't even need lube to pull it and more flexible than aluminum but more money
Too start over. Pull the current wire and replace with one piece from the main to the sub without splices. Of course 4 wire.
We could never get the 6" of extra wire in the splice box at the current cut lengths.