Thanks for your contribution to the pro-AGM argument.
We've chatted about the benefits of battery balancing before and we are in full agreement about its value in a series-string based system. You may recall I had one series pair of AGM's which had become unbalanced about two years ago. I tried to recover them by individual charging and I was planning to install a balancer in early 2019, but 2019 was a busy season and I didn't get around to it, and I've been unable to return to the boat since then. On detailed testing before I left the boat in late 2019 almost all of my batteries were very low on capacity. I don't know whether there is a causal link between the unbalance and the whole bank degrading. I inherited the bank from the previous owner, and I know it was installed in 2015, but obviously I don't know what use/abuse the bank may have been exposed to before I bought in 2017. This experience colours my thinking about the longevity of AGM batteries on the Amel boats, hence my expectation of 5 years. My previous cruising boats have all had less complex DC systems, none employing 24V, but (lead-based) battery replacement every 5 years was not unusual. Maybe it can be better, but I have not personally experienced that.
If I decide to stay with lead batteries for another cycle, I will definitely install the Victron balancer. At the moment I have 12x 12V AGM's organised as 6 strings with no interconnecting midpoints. My plan would be to introduce interconnecting midpoints to create two independent strings of 3 + 3 parallel connected batteries. This way I can use two balancers rather than needing six of them, whilst staying within the Victron recommendation that each balancer is able to balance 3x 200Ah pairs.
I like your idea of the alternator switch. I do have a 3-stage external regulator that changes from bulk charging to absorption mode at 28.5V, BUT it then holds 28.5 for a minimum 4 hours before changing to float at 26.5. Not the best if you are starting off fully-charged, but better than an alternator that sticks at 28.8V ALL DAY regardless !
It is an endless discussion as you say. Given the Benefit/Cost differential I think my decision will be either "Evangelical" (Go Lithium) or "Logical" (Stay Lead) :-)
All the best