How to prevent Garmin Connect from syncing when installing updates?

You can't stop that. The last update for 955 was two months ago so no need to connect it to Wi-Fi. That update was just bug fixes so unless you experience any issues then you don't need it. That is the easiest way to prevent the sync. Check the device forum to see when new updates are available and to see if you need to install it:  Forerunner 955 Series  

Why don't you want to sync your data? If we know what you want to achieve then we might be able to find a way to do that.

The problem is that I've had GPS issues in the past which (I think) were solved updating the software, so I try to keep the software up to date. If sync can't be stopped when connected to Wi-Fi, Is there an alternative way to install the device updates or at least the updated Ephemeris data? E.g. downloading a patch and then placing it on the watch when connected through USB?

Why don't you want it to sync the data? 

Privacy reasons.

Why not just make everything in your account private? Or do you mean that you never want to sync your data to prevent Garmin from storing it online?

To prevent your logged activities to upload to Garmin servers you can just remove them from the watch. Connect to a computer and copy all activity FIT files to the computer. Then delete them from the watch.

Yes, the latter. Besides activities there is other data (steps, HR,...) leaving the device, so I was hoping there would be a way to just get updates without having to upload personal data.