Is there a way to get Android Oreo (on a Pixel 2) to show the temperature in Celsius (rather than Fahrenheit) on the homescreen *without* enabling syncing of all "Web & App Activity" to Google?

The issue is that this screen doesn't sync appear unless you sync to Google?

Yes, at least based on the instructions I've seen for how to get to it (choose “Customize” in the menu of the Google app).

I do: Top right three dots -> Settings -> Settings -> Preferences -> Weather