I used two different versions of Miunlock. I used version 4.5.813.51 in order to get an SMS sent. I also used a Google Voice number. This version might work with any number, but I was well and truly down the rabbit hole by this point. I then switched to version 6.5.406.31 for the unlocking. Because cookies are involved, I was not required to receive another SMS code.
Before attempting to unlock, I made sure that I had a working android bootloader device (version 11.0.0.0 from Google) in Windows device manager by installing it manually and pointing the "browse for files" dialog to the drivers folder of the miunlock version 6. And then I found when it stopped halfway in the unlock, that I needed to boot back to normal, log out of the Mi account, log back in to the Mi account and then go to the Developer options and the dialog for check unlock status. After getting that squared away, booted back into bootloader, connected back to the version 6 (which I left open where it left off) and clicked "try again" and it worked. Oh, I also went to i.mi.com and did the find my phone routine because somebody mentioned they did that to get it all working. This may or may not be necessary, I don't know.