AGM batteries have worked fine in my car. I ran a made in USA Optima AGM battery in my '82 300D-Turbo - it lasted over 9 years. This was with the original alternator and original voltage regulator.
I replaced it with a Group 49 AutoZone Duralast Platinum AGM battery about 6 years ago. This battery has been great as well. The alternator was replaced with a new one about 2 years ago.
I use a CTEK MUS 4.3 Test & Charge to maintain it. It has a choice of 3 charging voltages. I recondition the battery about once a year as recommended. The last reconditioning cycle I ran took about 5 hours.
The CTEK user manual lists the charging voltages as 14.4 V for standard charging, 14.7 V for cold weather charging, which is what I would use if I needed to charge the battery with the charger; and maximum of 15.8 V for the reconditioning mode. The charger varies the voltage during various charging/reconditioning stages.
The user manuals are available here:
https://smartercharger.com/pages/manuals