In Catalina, all OS installed fonts have been moved to the /System/Library/Fonts/ folder. No font manager can disable any of them.
Font Book has had that little "gotcha" for years. It will let you choose Disable for fonts in the System folder, but then doesn't actually do anything.
There is only one solution for these fonts in Catalina. You must boot to another drive or partition with an OS on it, then remove the unneeded fonts from the non-startup drive.
Assuming English is your first language, there are a lot of fonts you don't need on the drive. You can pare it down to the absolute minimum as per the list in my article, Font Management in macOS. The required fonts for the System's Fonts folder, and Supplemental folder are listed.
Make sure to first save copies of all OS installed fonts. If there are any your third party apps require to launch or display correctly, you can reactivate the copied fonts with your font manager.
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Edit: Too slow! Barney-15E beat me to the link. 🙂