Well guys, for those who have problems booting/freezing up after upgrades to Big Sur or Catalina from previous MacOSes, I would not upgrade directly by just having a Time Machine backup. If you have enough free space on your drive (Catalina requires about 25gb and Big Sur wants 46gb to preform an install), I would create a new partition (Extended Journaled not APFS) via Disk Utility. Install the new OS onto that partition and test it out by booting from that partition. I have High Sierra, Mojave, Big Sur on separate partitions. You can always resize them to include more space if you have. At least this way you have a bootable machine. On High Sierra and Mojave, just renaming the AppleThunderboldNHI.kext did fine. But on Big Sur, I followed the instructions, it still freezes. Still trying to figure out why. Haven't tried disabling the Firevault. Can someone help me with this. Thanks. Another thing if your drive's name has a space in it, need to put it in double quotes like Big Sir should be /Volumes/"Big Sur"/ ... . If you don't have enough free space on you SSD, use an external USB drive, create a new partition on it and install the new OS onto that partition and test it out. Run all the commands on it to see if it is stable. It will be slower since it is on an external drive. Just be careful when creating and deleting partitions, once deleted it is NOT RECOVERABLE!. Good Luck.