Hmmm, you reminded me of something similar that happened a few days ago during installation of 2020.4.1. There was no problem downloading, but when the MX went to install the update, it took about 45 minutes to get to 30% and then it stayed at that percentage with a "updating auto pilot" message for about 5 hours. Then it went to 60% and stayed at that percentage for another 6 hours with the message of "Updating Electronic Control Units" and then it finally finished installing.
So 11 hours for this installation which was a surprise. Yea, I'm sure there are many people getting it installed in under an hour, but it left me wondering why this was taking so long on my car when all of my previous updates (about 30 of them) generally would take anywhere from 40 to 50 minutes. I hope that was just a fluke.