losslessly convert your cbr files to vbr (someone help me out with the name of this tool).
Honestly, I wouldn't even bother with lossy-to-lossy transcoding, I would simply buy more storage space. It's rather afforable, considering how compressed is MP3 already.
The next thing you should do is buy more storage space.
I have no preference for any (vorbis, opus, aac, mp3 etc),
so it doest matter the codec i choose in this case?