I hope this helps someone and isn’t just a coincidence.
Yesterday, I woke up and my watch was dead which was odd since I had pulled it off the charger after 11pm the night before. Normally, it had almost 90% charge left at this time.
I charged the watch back to 100% first, and then saw that there was an update and thought that would fix the battery drain. So, I tried to install it and it got stuck at the “preparing” step and I found myself in this thread looking for a solution.
I tried a factory reset and re-pair and that didn’t work.
I then found the recommendation to delete the download and try again. I also restarted my watch and phone each time I deleted the download. I did that 3 or 4 times and it kept getting stuck at the same point during installation of the download. I had hoped that somehow the battery drain problem would fix itself, but it didn’t. I had other stuff to do so I deleted the download and went about my day.
I tried again, about 6 hours (and over 50% of my battery) later and the update stalled again (after I charged it above 50%). At this point, I decided to turn off automatic updates for my watch (which I’ve always kept on) and then delete the download one last time and restart my watch again.
Since that time, the battery drain has STOPPED.
I don’t have the latest update, but at least my watch will make it through the day now.
My conclusion (and it may be wrong but hopefully not) is that some combination of this particular update plus automatic downloads is the culprit to the battery drain.
Turn off automatic updates, delete the download, restart, and cross your fingers.
Good luck!