So am I stuck with sending this thing in with stock FW, even though it's S19 on an S21 lol
You can't use .bmu file for SD card flashing. All SD/TF card image for newer units was removed from bitmain it's no longer available use the link provided by mikeywith above make sure it is .zip or .rar file.
You will need an SD card to flash the unit; make sure it is formatted in FAT32, and then extract all the firmware pack files onto the card. After that, plug the SD card to the unit then run the unit. 2 led lights will blink and it would stop when flashing is done.
Make a factory reset by holding IP report/reset button for 5 seconds then release.
Then update here and let us know if reverting it to stock firmware is successful.