FLAC is a lossless codec, so compressibility is limited.
At best, you can use
ffmpeg -i "video.m2ts" -vn -sn -acodec flac -compression_level 12 "audio.flac"
Since you have a MPEG transport stream, you can also try
ffmpeg -i "video.m2ts" -vn -sn -acodec copy "audio.mp4"
This will extract the audio without re-compression.