Does anybody use solid wire on a robot?
, By the time I get to #6 wire I am generally using lugs & not twist connectors. The Ideal twist connectors (wire nuts) are UL listed for solid and/or stranded wire. In building wiring it is probably the easiest way to make the transition.
I agree with Al that you want as few as splices as possible. Every one is a failure point. Whatever ever method you choose make sure it is done properly.