Davx registers for Autostart, that should not be the problem. What you should do, though, is switch off battery optimization, that’s important. Davx runs fine for me with my nextcloud instance
Davx has created the account to access my events on the cloud server
But when I ask him to synchronize with Goggle Agenda on the FP3 , it says “Synchronization starts” the nothing else : no indication that it is working and no event in Google Agenda
You might activate logging. But then, does it work with Google agenda? Google’s known for using its own protocol, and no standard. As i can see it’s not listed in the “tested services”