I keep forgetting that I can't reply to the community emails you send. Sigh.
Since granting Mp3tag full disk access pretty much everything now works. My observation was that most apps request to be granted disk access if they fail to open a device for read or write. I've never seen Mp3tag pop that dialog (including an offer to startup the Security and Privacy preferences.
I am doing this on a Intel Mac running Catalina. That's where most of my storage is attached. I have a M1 MacBook Air (Big Sur) that I also run Mp3tag on just for playing around with features.
In general I wouldn't grant Full Disk Access to an application that didn't strictly need it. However since I have music storage split across multiple devices I have to grant access or I'd be forever running into restrictions.
My iTunes storage is a partition on a Drobo. CARMUSIC is a partition on a USB drive. Finally the thumb drive is used to move music to the car.
FWIW BMW infotainment systems are very fussy with regard to tags. Any image over 500 square pixels will not display. I know JPEG images work, but I have not experimented to see if PNG or other formats work. Any file with an unsupported extension (regardless of content) is ignored and the entire directory containing the unsupported file type is inaccessible from the iDrive UI. At least my current BMW supports a variety of formats including flac and m4a (mp4 is unsupported obviously). The previous car, which I owned while starting to use Mp3tag, had very limited file and format support.
Without Mp3tag the entire process of moving thousands of files to the iDrive environment would have been much less rewarding. I know. I tried.
Thanks again, Florian.