convert: Granular control over which formats to convert, especially in auto mode

I'm trying to use beets to automatically recompress my FLAC albums before adding them to the library. It's a FLAC to FLAC conversion. I'd like to use the maximum compression ratio (`--best` option). Currently there's a `convert` plugin for beets that seems to work only with music already in the library.

Is beets the right tool for the job? Or should I manually recompress music with some custom script before adding to the beets library?