The good news is that I have until Spring to figure all this out. I don't like my current 4d batteries because they are big, heavy, tend to leak when the boat heels over and are surprisingly expensive, all things considered. Replacing them with batteries with similar issues is also undesirable. In any case my new batteries will need to fit within the current battery boxes. From what I see advertised on Amazon, I do not think 4 6v golf cart batteries will fit in the available space. I could simply get 3 type 31 AGM batteries. But, for not too much more money, I can get a greater capacity with other alternatives that are smaller, lighter and last longer. As for what I "need," well all things being equal, fewer, smaller, lighter, longer lasting batteries are better than more, bigger, heavier batteries that I have to charge and replace more often. There is not much on my sailboat that I actually "need." Somedays I wonder if I "need" a sailboat at all.
I hear that if I simply connect the Fireflys, as I would other Lead acid batteries, I risk ruining my alternator and only getting a short life out of them. The impression I have gotten, from has been written in this thread, is that these batteries are just as picky as Lithium batteries. But, I wonder, how tolerant these batteries really are to inaccurate charging? Unfortunately, I do not get a sense of this from what I have researched or seen in this forum so far. The unit to unit output of real world battery chargers varies to some degree. I doubt if batches of actual batteries are identical. There certainly has to be some leeway, otherwise these batteries would be totally useless in practice.
So I wonder, what is the cost of a mismatch between the stated charging requirements and the real world consequences of them? Would they really kill my alternator? If it is just a matter losing 10% of the life of the battery by using the baked in settings of my current charger(s) and occasionally allowing the alternator to charge them? In that case, Firefly batteries would still be highly attractive. If the Fireflys are really no more tolerant than Lithium batteries, I see no advantage to them. I would still have to isolate my house from my starter battery in either case. If that is true, I just see no point to buying them, since Lithium batteries have come way down in price.
Vendors like to sell expensive equipment and much of the information comes from these same vendors. In my 16 years owning a sailboat I have learned, the hard way, that one is well served by a bit of scepticism. Marine equipment is expensive; I do not want to replace perfectly functioning equipment. There is no limit to the amount of money a boat can absorb.