Very interesting...You said "One of the key parameters for us is to watch the difference between the “high” and “low” halves of the battery bank. If they drift apart from being identical (half the nominal 24V total) that is an alert to a problem."
Do you not have a balancer across the 12V battery pairs to keep the voltages the same?
We have 3 parallel strings of 4 x 6V AGMs in series with balancers across the 4 and the voltages on each battery stay the same within 0.02V and have done now for 4 years.
