Wow. That is quite a project you've got going there. Lots of variables.
, an AGM will be okay on a wet-cell charging profile, but it will not be optimum. Which is not to say that it's okay to mix chemistries -- only that you might be okay with the current alternator/regulator when you switch to AGMs. The usual recommendation is to not mix battery chemistries.
But a project of this scope is well beyond my pay grade.
Hopefully some of our electrical gurus will check in (e.g. btrayfors or Mainesail) and maybe offer up some advice/direction.
Anyway, sounds like you've i.d.-ed the starting battery.
Best of luck.