Have you read the FAQ I posted on the trickle charger.
https://community.victronenergy.com/t/how-does-the-multiplus-trickle-charger-work/98
I would not be worried too much about keeping the trickle charger on permanently from the AGM perspective as it will see around 13.2V which is a storage charge voltage. The issue is making sure the trickle charge is stopped on low lithium voltage which depends on your battery/BMS. Putting it through the Cerbo relay is certainly an option, or a programmable relay on the Multiplus.