TL;DNR: Pairing can take a *long* time if you have a bunch of Strava segments being sync'd.
Thought I share this in case it helps others avoid wasting half a day for no reason.
I have a fenix 5 that was currently running the 6.71 beta firmware (no particular reason, just my typical geek curiosity) that I needed to pair with a new Pixel 2. The pairing would begin as expected but the progress bar would quickly get to about 2/3 complete and then stop. After several minutes with no change, I assumed the pairing was not working and would cancel. Mistake #1.
Thinking it could be something with the phone, I tried pairing re-pairing it with the old phone. Same result so I assume it's a bug in the beta firmware. Mistake #2.
Even though I knew I would lose all my settings, I decided the easiest fix was to revert back to version 6.0 which was included in the 6.71 beta. Apparently I had already trashed the zip file and it was no longer available via the Garmin download page. Mistake #3.
But lo and behold, 6.75 is available. Maybe that fixes the pairing bug. I'll give that a try. But not paying attention, I download the the firmware for the fenix 5S when I have a fenix 5. As it turns out, dropping the gupdate.gcd file in the /garmin folder just quietly fails. Mistake #4.
After realizing I had the wrong firmware and downloading the correct one, I was able to upgrade to 6.75 but the pairing still hangs with the progress bar at about 2/3 complete. OK, lets go ahead and revert back to 6.0 now that I've got the correct archive. Drop the backup gupdate.gcd into /garmin, reboot, downgrades to 6.0, try to pair. Same result.
At this point, I'm just trying different ways to do the pairing and, unfortunately, I have no idea what I did, but I wound up on a display that gave a more step-by-step status of the pairing. The final step had a progress bar with a label saying something like "downloading 1 of 77 updates". I had no idea what these updates were, but the number would update (albeit slowly) so I just let it run. Eventually it completed and the watch was finally paired. As long as the watch is back to factory settings, I went ahead and bumped it back up to 6.75. Fortunately, all is good.
Later that day I went for a run and at some point the watch did the typical "approaching segment" display. That's when it finally occurred to me what those 77 updates were - a bunch of Strava segments marked as favorites.
Moral of the story: be patient when pairing if you're using a bunch of Strava segments. Might be better if the segments downloaded after the pairing and during the first regular sync.