What can limit charge output from the converter?

[Wmlunt]

Temperature plays a role in the conductors as well. As the smaller wire heats up the conductance and resistance changes, which pulls more current, self failing propagation.
I keep threaded connections and number of connections to a minimum, good electrical designers consider every connection to be a potential failure point. Threaded connections in our RV vibration environment will loosen, lock washers don’t lock. Split bolts don’t provide a gas tight connection and larger cables hand crimped, require tooling the average user doesn’t have, and stil don’t really crimp as well as machine crimped.
Multiple cables purchased with crimped ends require more terminals to be stacked on the threaded bolt, more resistance and vibration issues.
I personally would use the right gage single wire, or increase the size if desired. Seems the lowering the resistance fools the converter, maybe getting a programmable one to change the charging profiles may be a better choice.
In my case of MM2108 not sure it makes a big difference in the real world. I left my inverter on by accident and pulled batteries down to 20%. Went back the next day and 200watt solar panel had recharged them by lunchtime to 100%.

[eatSleepWoof]

A small update/data point: I found that when my batteries are discharged "deep enough," the converter DOES allow much more charge current to go through. With the batteries at 28% charge, I saw a charge of 35.5a going in. By the time the batteries got to about 68%, the level of charge dropped back down to the "usual" 10a. This does point to the charging profile being the limiting factor.