Couple questions here.
My last vehicle also used AGM before I dropped in an aftermarket AGM battery, and both of them I used to once in a while charge up. I used to use a really old school, but regulated slow charge that would max out at 2AMPs and I could set the regulated voltage to less than 14V. Isn't the trick with AGM to slow charge and never allow the charging voltage to get above 15 or 16V?
Also, I see mention of Optima a lot in here. FWIW, I used to sell and use Optima's in custom cars and car audio/video systems in the 90's (e.g. built a suburban years ago with 6 stroker 18's, 12 Optima blue tops, 4 1uF caps, etc) and the batteries were king then. After that, they were acquired and manufacturing was moved to Mexico. From there on, the quality went downhill fast. We would swear by that brand, as I understand it, not one of the shops I used to work at even sell the brand anymore. In 2014, I bought a red and a blue top for a dual battery system on a Jeep, thinking they were still one of the best. Both were effectively dead under 18mo (and for those that don't know, on those Jeeps the main battery is stowed inside under the passenger seat). Was really disappointed. Maybe the chargers are better than their batteries but I won't pay a premium for them. I do see a lot of great reviews of Bosch batteries but man they are expensive.