There's probably little doubt that attaching the extra 300W of solar directly to your battery bank did them more harm than good. The constant 28.8V from the alternator doesn't help either if you do long periods of motoring.
Our 12 x 6V AGM batteries came with charging ranges Absorption 7.25 - 7.45V, Float 6.8 - 6.9V.
I opted for the middle of the range so for 4 in series as a 24V system, we have Absorption 29.4V and float 27.4V.
All our charging systems (Engine Alt, Solar, D400 wind, Victron 100A and Promariner 30A) are set to those voltages and all bar the D400 wind generator have a battery temperature input, so those voltages vary depending on the temperature of the batteries.