Solution to battery drain? (New issue for me)

I've seen lots of posts on battery drain, and given them a cursory look but have always been happy with the life on my Fenix 6X Sapphire.

Until now.  I don't know what's changed in the last couple of weeks, but it's getting absolutely ridiculous now.

The original battery monitor app I was using had me at 0.7%/hour, which is a 6 day battery life (no GPS activities)

I did a soft-reset today (the one that keeps your activities and music but clears everything else), and put Battery Gauge on and charged to 100%

Now it's on 68%!  In 5 hours!  With a run that was 1hour 8minutes long.

In comparison, when I first had the watch I ran a 50 miler in 10 hours with full GPS and the watch went from 100% to 80%.

Is there a solution to this issue?

I'm assuming it's a sync problem with Garmin Connect as that's the only part of the link that could have changed (I'm on the latest non-beta watch software but that was released in January and the battery problem has only been apparent for a few weeks).  Having said that, I turned off the Phone connection yesterday for about 8 hours and it didn't seem to make a significant difference to battery drain (hence the reset this morning).

I do long events - 10 to 36 hours - and right now this watch will be flat before the end.

Any help much appreciated!