I too am a Sonos user. I have not used the B2’s mirror file feature. One of the reasons I avoided it was that duplication in the Sonos library was an implication as the MP3 mirror files are held close to the main WAV/FLAC files in the B2’s directory structure.
As far as I can see, there are three possible fixes for Sonos users:
First, do not use the B2’s mirror file feature. Instead, create MP3 copies outside the B2 environment using a third-party tool
Second, ask Brennan to move the location of the mirror files somewhere other than the music directory
Third, upgrade alternative music players on phones/in cars to use WAV/FLAC formats rather than MP3 format, or use streaming services on phones/in cars, obviating the need for the MP3 copies.
I have used the third option. The only downside of this option is that fewer files can be stored when disk space in limited. On the other hand, it avoids the need for mirror copies. WAV/FLAC files can be extracted from a standard B2 export. IMHO, streaming services are the best option for use on phones/in cars or anywhere else where accessibility rather than sound quality is the most important feature.
PS I don't think that Peter’s suggestions will work.