I too suffer from the Big Sur failure to finish uploading, every time I try, I can’t get it to finish continuing to upload. I’ve tried the ins and outs of ever hard reboot, soft, I even pulled my cord out of the socket, when I slowly put it back in and pushed her button it booted up again and it gets to the last moment before fully finishing and it just stops. I’m sure the other end is happy as hell but I just can’t be satisfied until I finish and she boots up perfectly, fully connected running 500mbps, information flowing fast af, back and forth, in and out, and I know everything is fine when im all over my task and finish and then when I come back to finish another… when everything goes right that time, that’s when I know it’s fixed, sometimes a hard reboot and a yank on the cord will help anyone finish its start up load.. the release of all the anxiety feels sooo good when you get a happy ending. Then I can relax and smoke a cig.