So my question is based on the following:
You originally brought up 30a. This would have nothing to do with the battery. Ohms law would put 30amps at over 300amps using an inverter to supply 115v current would be how such a demand would show up.
You ask what wire size from shunt to battery, later on asking about two batteries (a bank).
A 2 or 3 ft battery cable is cheap!
This would be my solution, I figure sooner or later you will install an inverter, (most people do).
Parts list:
2 @ short battery cables.
2@ terminal posts.
1 @ battery cable = in length to the above 2 cables.
Mount shunt on panel close to battery and run short cable between battery to shunt.
Mount terminal posts to panel, where easy access is available.
Run second short cable between shunt and terminal.
Run longer cable from second terminal to other battery connection.
Connect all your present wires to the new terminal posts (be sure to color code your posts.
The sense wire is good at 22awg or whatever wire you have on hand.
Sorry for the delay, had a market run today.