I try to avoid soldering when I can.
For your setup, it would be possible to crimp the various ground wires into a single connector (most of my ground wires up being about 18 gauge, depending on the component) and then grounding it to the mounting points on the m-unit. Generally, I would take all the ground wires from the front-half of the bike and ground them to one side of the m-unit and the ground wires from the middle of the bike (R/R, ignition unit, etc) and the rear of the bike and then connect those to the opposite grounding point on the m-unit.
Redundant grounds are nice to have in an OEM setup, but they sure make things more difficult to troubleshoot when you're doing your own wiring.