I advocate leaving an AGM on charge when not used. However, you need a good quality charger to do that. In my current situation, I have mains power to the van, the charger floats the battery at 13.2 V and once a month puts it into a brief full charge cycle to condition it.
However, I'm moving home in a few months and the van will be in an area without power access, so I'll leave the solar panels on. The Victron has been reprogrammed to float at the same 13.2 V.
If Jaros's battery lost 5% a month = 6.5 AH, the charger will recharge it. If he left it 6 months it might lose 39 AH, his charger will still charge it, but just take longer to bring it back to full, compared to say my charger. I agree, his charger could be bigger and would be better if it were, but, what he has will work.
I do not dispute the available power in a comparable Lithium battery, or the weight saving. It all boils down to what you have, what you need and how deep your pockets are.
I am looking to put a 100AH Lithium into my caravan before I next use it. However, my mains charger will be ok as it a. has a lithium profile, but I will use power supply mode. and b. my mate will reprogram the Victron solar controller to suit. I don't use DC-DC in the van.