12.4 volts in an AGM is really pretty low.
As a general rule 16+ volts while charging will substantially weaken, & given enough time, kill an AGM battery. What type of battery do you have?
Inoperative headlight was a clue.
My tractor had what I suspected was a bad starter relay. I used a small 300 amp jump pack to bypass the relay & used the on-off switch on the jump pack as the starter switch. Turned out it was a bad seat safety switch, easily bypassed
