If your lifelines are that weak, adding a group31 to the mix will just have it feeding the greedy lifelines and not provide much for your overnight needs.
Separating all the L-16s and taking resting individual voltage readings should indicate which two are the weakest nd you can try the other two to get you through the trip.
I'd not waste any more time or generator gas trying to condition the 4 in series parallel. It is no guarantee that there is one bad one dragging down the other 3, they all could be shot.
Regarding getting more AGM or going Lithium, lithium are not just plug and play. One overcharge or overdischarge can ruin them.
The Firefly Oasis carbon foam AGMS are pricey, but said to be resistant to PSOC cycling, and now come in an L-16 size. I am personally still on the fence regarding them. Many people primarily in the marine circles, seem to think they are a good option instead of lithium as no existing charging sources need to be replaced and they are more or less plug and play.