MtnDon wrote: two 6 volt in series is better than two 12 volt in parallel, even if the batteries are the same design. When in parallel, a small difference in internal resistance will cause more current to flow through one of the parallel batteries. That can result in one battery being overcharged or the other one being undercharged. The best lead acid based battery off grid systems, for example, avoid parallel batteries as much as possible.
Mostly right in your above post, but this part is a bit overstated.
Going past 3 parallel yes, but just a pair, balance issues not that significant.
If AGM is required, and especially high-current use like winching added to normal house use, a **true dual use** 12V TPPL unit like Odyssey PC2150 will give best of both worlds, and 2-3 paralleled is OK to increase the AH.
Just need to call them to locate a local source, don't ever want to pay to ship lead if you can help it.
And never imagine an AGM "marine deep cycle" from auto or big box sources should even be discussed in the same breath, really the FTC should prosecute those for false labelling.
https://marinehowto.com/what-is-a-deep-cycle-battery/