Shamrock has the OEM setup with 2 4D batteries. I get 5-7 seasons from the Deka batteries, change them when I can't get 3 days/night at anchor, ie no charging, running fridge, lights, keurig coffee maker and microwave. If I spent every night at a dock could probably get a few more seasons.
Check your charger that you can set/program to standard wet cell batteries.
What I understand the advantage of AGM batteries is that they will accept a charge faster. Is that important to you?
There is always a lot of discussion on batteries and how to setup and use. IMHO is that it really depends on how you use your boat. Do you sit on a mooring or at a dock with shore power. Can you sail quickly or do you need to motor to go sailing. and more considerations.
Jim