While lossless formats (FLAC, WAV) are great for preserving original quality, using them for a local music collection can result in significant usage of storage space. If that's undesirable, using lossy formats can solve the problem.
While choosing between many lossy audio coding formats, transcoding to the Opus format in the VBR mode with highest compression level is a good way of having the best quality at the smallest file size.
Lossless audio files can be transcoded using FFmpeg with the libopus audio encoder. The default settings for that encoder have vbr set to on and compression_level set to 10 (the highest).
Thus, it is enough to call
ffmpeg -i "file.flac" -c:a libopus "file.opus"See also my script with an example of size difference for a release.