Hele Battery in non - Hele car

First of all buy yourself a battery tester where you can measure the State of Health (remaining capacity). The battery voltage may be fine, but over time the capacity will drop, which can result in the issues you describe.

This one from Amazon will be fine to give you an 98-99% accurate indication on the SoH of your battery.

I would always replace the AGM with AGM and non-AGM with non-AGM. I would not replace with a genuine Ferrari, but find a good quality. There are many suggestions in here on which batteries to get.

I think the non-AGM is a 12V 100Ah 850EN, so find a 12V 100Ah with a EN as close to 850 as possible and you should be fine, and then remember to do the proper restart procedure when the new battery is in.

 

AGM batteries have a lower internal resistance than conventional wet lead-acid batteries so you need a specific bench charger that charges at a lower current/longer duration than old style chargers.

Apart from that, everything I have read says you can put an AGM battery in a car that originally came with either a WET cell or AGM battery, but not the other way around. If the car came with AGM you should not "downgrade" to non-AGM.

 

Your assumption that you are charging at a slightly low Voltage level is 100% accurate. The car's entire "system" is tuned to the battery voltage range and a wet battery system (alternator, internal regulator, resistance, coils, etc) are set to that level. The AGM battery should perform but not at its peak level of cold cranking Amps.