How to prevent Google from "synchronizing" my personal data after a factory-reset, adding a new Google account or software updates?
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.