I think those voltages considering a 8 to 9 amp load are fairly low, , and the rebound to 12.6 is certainly better than not rebounding that much, but could easily be indicative of a sulfated battery.
I'd try and restore capacity/desulfate by draining to to 12.1v rested, under a 10 amp plus load, then charging it at 25 to 30 amps until 14.7v is reached, and hold it until amps taper to 0.5% of the amp hour capacity which might take 4 to 12 hours. Perhaps even longer.
Once battery cools I bet it would hold higher voltages under the same loads.
How much of its capacity will actually be restored is an unknown.
If the relatively high amp recharge from a well depleted state does nothing, I would then boost voltage to 15.5v after the regular full charge, for several hours, monitoring it closely for excessive heating.
My Northstar group 27 AGM, 930CCA, 90Ah capacity will, at 700+ deep cycles and 4.5 years of age, easily maintain 12.6v+ under a 10 amp load until it depleted 15 to 18Ah of its original 90.
its full charged resting voltage used to be 13.06v. Not sure about now as it never gets a chance to rest, but this particular battery loves the high amp recharge from a well depleed state and It always sems to restore performance, which I judge by voltage held under X amount of load and X amount of Amphours removed, and much observation of these variables throughout those 700+ deep cycles.
