Hey Joseph Becker,
Welcome to Apple Support Communities. If we understand correctly, you're having some issues with Audio Unit plug-ins and Final Cut Pro. Right?
This guidance from Where are third-party Audio Units plug-ins installed on Mac? - Apple Support may help to get Final Cut Pro to recognize the plug-ins.
"You can install Audio Units plug-ins from manufacturers other than Apple to process audio in Logic Pro, GarageBand, and Final Cut Pro. When installed, Audio Units plug-ins appear as individual components in the Library folders on your Mac:
- In the Finder choose Go > Go to Folder, enter "/Library/Audio/Plug-Ins/Components" into the Go to Folder field, then click Go.
- You can also check the Library in your Home folder. In the Finder choose Go > Go to Folder, enter "~/Library/Audio/Plug-Ins/Components" into the Go to Folder field, then click Go.
Built-in effects and instruments included with Apple apps like Logic Pro, GarageBand, MainStage, and Final Cut Pro don't appear in the Library folders and can't be removed manually."
If not, reach out to Apple Support for a closer look at the issue.
Take care.