If your Symphony I/O MkII Thunderbolt is not recognized in Symphony Control application (in macOS Big Sur/Monterey), then this is the guide you want to follow along.
1. Confirm that Symphony is recognized in PCI and Thunderbolt in > About This Mac > System Report > Hardware, but not in Audio (as seen in the screenshots below):
2. Open Activity Monitor in Finder > Applications > Utilities, then search "Apogee" in CPU to find "Core Audio Driver (ApogeeSymphonyIO2T.driver)":
3. Double Click "Core Audio Driver (ApogeeSymphonyIO2T.driver)" then "Force Quit" the process:
4. After "ApogeeSymphonyIO2TSymphony.driver" restarts itself, Symphony Control.app will recognize Symphony!