If not, any alternative ways to sync them automatically?
I prefer the layout and cleaner UI on POLAR Flow when compare to Garmin Connect. But the Garmin watches works better for me overall.
Polar pull data from Strava?
You better look at Polar. This is a Garmin forum, so most people here know nothing about Polar. If you are lucky, perhaps someone familiar with Polar comments later, but you'll get the answer much faster when you ask at Polar (or at Strava).