For comparison, Trimetric says shunt to meter cable can be up to 300ft if #18 or larger, or 100ft of #22 wires.
Not clear (I didn't look closely) if the OP meter reads both directions. Some do one or the other depending how wired.
Lots of other things have to go on the shunt besides the inverter neg for size of bolt issues--use a neg buss bar to get around that problem. The wire from buss to shunt needs to be fat enough to carry the total amps of all the "branch" wires going to the buss. Which would be at least as fat as the positive wire to the battery.
