Solved!
I changed both Starter Batt (+) → Orion XS IN and Starter Batt (-) → SmartSchunt (LOAD), and it works!
The old cables were replaced with new ones because the cable resistance caused the voltage to fluctuate, leading Orion XS to think the batteries were fully charged.
Thanks marcvdijk for the inspiration posted in Brand new Orion XS not working - #8 by marcvdijk