As for the auto-sharing - there is a simple solution for that problem, and I use it too. My default setting for new activities at Strava is "Only You", and I set it to "Followers" only once I adjust it, and decide to share it.
As for the comment and photo - that is rather a request to Strava. Garmin does not push anything to Strava. It is Strava who reads the data from Garmin's API, so the request should go to them, and they can then contact Garmin if they need some assistance from them.
That's a not a solution
That's exactly what the OP asked for - a delay before the activity goes public, allowing to edit the details and comments before publishing.