How to prevent Google from "synchronizing" my personal data after a factory-reset, adding a new Google account or software updates? It seems that after a factory-reset, whenever a new Google account is enabled on a device and potentially after some software updates sync will be enabled again even if it was turned off in the Android settings before any personal data was loaded onto the device.

How can this be prevented?
The approach(es) I'm using:


don't use a Google account at all (if really needed, Playstore can be accessed via e.g. YalpStore – and instead of GMS I use microG anyway). As that's not practicable for some folks:
before adding the Google account, make sure you've prepared a different "account storage", made it the default and have all your data (contacts, calendars) in there. Examples for that:


DAVDroid with some self-hosted server (e.g. ownCloud, Nextcloud, Baïkal …)
"Device storage". Either your device/ROM supports that out-of-the-box, or you've got to use some helpers like Offline Calendar for calendars or MyLocalAccount for contacts.



I've personally made very good experiences with DAVDroid. Google Sync never touched its data.