For anyone away from their gaming PC, some info to set expectations:
By the time I rolled up to my PC to check on things at 8am this morning, the Epic install had already completed its update and the manually-triggered Steam update took roughly 40min to get 35.9GB. My Tweaker-managed MS Store install has been somewhat problematic--there have been a couple of false-starts where it seemed to forget what it was doing and I had to restart Tweaker and manually trigger a file check to get it going again.
I turned on my XBone within the last 30minutes, and it looks like it wants to re-download the entire 79.45GB. There's a "Ready to start" marker on the progress bar around the 6GB mark... The icon updated to NGS when I tried to start PSO2 and got the needs update prompt.