I had one battery out of four house batteries die just before leaving Cape Verde for Barbados. I removed it. The other 3 x 110 ah batteries were fine. I replaced all the house batteries when we eventually reached Martinique. What I learnt was that I had to ensure more frequently that the batteries were charged up when they got below 80%, but the bank of 3 didn't take as long to fully charge.
You didn't mention whether you still have the original Santorin charging regime or if you have added solar, wind or gen set. Also will you operate mainly from a marina with shore power or self sufficient anchorages.
A bigger bank may last longer, but takes longer to charge to 100% - important if you want maximum life from the bank.
Just another dimension to add to the decision making.
All the best.
Ian 'Ocean Hobo' SN96