Sync initiated from the watch still hangs

However, I thought that the "sync" feature would sync the watch with Garmin Connect, instead of Garmin Explore.

If I manually launch Garmin Explore on my phone, then launch sync from the watch, it works.

If you use an Android phone, there are plenty of tips for fixing connectivity issues, in the Sync / Connection Troubleshooting Guide. Unfortunately there are far too many Android mutations, and each manufacturer is introducing own modifications which often screw up things, so the list of possible problems is really huge. I'd recommend to start with checking the Device Sync Audit in Garmin Connect - it may perhaps give you some clues what's wrong.

If you use an iPhone, the troubleshooting is much simpler - missing app permission are the most common culprit.

Same behavior for me (thanks for the tip about Garmin Explore). Synced quickly when I opened Explore, never with Connect.

What phone are you using? Brand and model?

Using a Google Pixel 6a