It turns out that the UConnect is very picky with compatible flash drives.
I actually threw my files onto a microSD card, and then used a microSD-to-USB adapter. It kept failing and backing out of the update. I tried a different flash drive, no difference. I tried a different microSD card, no difference. Finally, I remembered that on my 2020 with UC4, it was picky on the types of USB flash drives you used for updates. So on a hunch, I used the same/original microSD card, but a different microSD-to-USB adapter. It worked on the first try.
View attachment 162435