Finally got it figured out -- @bcopeland, hopefully this can be helpful info for you...
TL;DR -- if you're updating more than one device, you need to do a hub restart between each attempt.
My goal was to update 2 Zen15s and 2 Zac36s. Just one Z36 is paired LR, the other 3 devices are paired mesh S2.
I started with the non-LR Z36 -- that update went very smoothly.
As documented above, my next attempt to do the LR Z36 was rough... The progress was glacial (~1% every 10-15 minutes) and first attempt failed at 75%. Ran it again starting early yesterday morning -- same glacial pace, but this time, it did successfully complete sometime late overnight.
So this morning I tried the first Z15 (S2 non-LR) -- this one went great too -- fast & smooth. Then I tried the second Z15 (S2 non-LR), but it was that same glacial pace again.
Then I remembered that I updated my hub early this morning (to the latest beta) before I updated that first Z15, so I decided to try another hub restart...
After restarting, the update of the second Z15 went slick as snot too!
After that first LR Z36 kerfuffle, I assumed the wonky update was LR-related, but I don't think that has anything to do with it -- it's actually just all about doing a reboot in between attempts.
It's been a very long time since I've updated back-to-back devices using the built-in updater, so I honestly don't remember if this restart thing has always been a wrinkle or not, but hopefully this info is helpful to Bryan and fellow users.
