I'm not using Big Sur on the ProTools machine, but it's possible to do an internet recovery and choose an older OS. It's been a while since I've done this so I don't remember every step, but I believe I had to download the Catalina installer first, then it saw that during Recovery and allowed me to select it. Here's where you can get older OSs (under Download Mac OS):
https://support.apple.com/en-us/HT201372