Strava up load

So, everything is uploading correctly to Garmin Connect?

The transfer to Strava is from Garmin Connect servers to Strava so the app or Garmin device got nothing to do with the transfer issue.

Does it work to manually import the rides into Strava?

If it hasn't worked for a few days I would disconnect Garmin Connect and Strava and connect them again. Make sure you do everything on the web and not in the apps on the phone. Also use the normal username and password login for Strava.

I have completed my first Zwift ride this morning having recently bought a 1040 and Fenix 7X; it synced to Strava immediately and Strava immediately synced with Apple Health.  Still nothing in Garmin Connect - I have disassociated Strava with Garmin and then reconnected and still no joy.  I tried connecting Swift directly with Garmin, no syncing directly from Swift either.  So frustrating.

Activities from Zwift are not synced to Strava. Instead you should sync Zwift to both Garmin Connect and Strava.

"NOTE: Garmin Connect will not forward data that has been received from one third-party fitness website to another third-party website that is also linked to your Garmin Connect account. This aligns with the policies of other third-party services and helps to prevent duplicate data from occurring across accounts."

https://support.garmin.com/en-US/?faq=lmXLT2CHNO6e2Bj26B5aQ7 

Understood.  Will disassociate Zwift - Strava wasn't syncing before I did this though (the activity wasn't synced for over 2 hours before linking Zwift)

It's now 4 hours after the activity and it has still not synced.

Understand and did all as suggested with no luck to date..

I'm not sure if the old activities will sync. They should sync the first time you connect them, but not sure what will happen when you disconnect and connect again.