OK tell me what is wrong with my plan?

I want to add power to my 326 aft simply for a little extra longevity on the hook to run lights in the evening, perhaps and couple hours of tv in the evening and make a cup of coffee in the am. My boat is located at Bayport in Midland and I have had two quotes to do this professionally 1 quote was to add 4 - 6V's with inverter, new charger etc and the installed quote was just shy of $10K. So I then had them quote the above with 4-6V's with separate charger etc and it was $5K. Way too much in my opinion. The quote included isolating the 2 - 12v's as starters only as well which currently these double as starters and house. Boat has a separate starter battery for the Genny so I am fine there.

My plan is to add 2 - 12V's to the existing ones so I will have 2 on each side to double as starters and house and I plan to add 1 - 6V under the counter in the kitchen and run an eliminator 2000W inverter simply for power to the TV and coffer maker when needed which will also be located there. Would have a separate smart charger to charge the 6V when plugged into shore power or running the Genny.

I know this is a little bush league but based on the quotes i have received this do it yourself plan is far more economical and 1 that I can tackle.

Other than the obvious that I should have separate house/starters any potential problems???

Thanks in advance