I just replaced the Alternator on my Ford Explorer . . . $600 for the part + $200-ish in labor
So, not taking any chances with the boat engines. I figure that I can go with more minimal lead-acid batteries, and as needed, there is a 'parallel solenoid' with a helm switch to combine the starting batteries during cranking.
Here is my current & proposed schematics.
Current System: (OEM + my modifications)
This would reduce the weight by about 350 lbs. I plan on having the LFP batteries more forward in the boat, rather than in the engine bay (my understanding is the the LFP batteries don't like heat (or extreme cold) ).
Pricing this out, it looks more do-able as all the bits and pieces would run about $2,000. Probably will be a project for over the winter 2023-24.
Thanks everyone for your comments so far.