When you convert audio from a lossy format to a lossy format (from YouTube's native AAC streams to MP3,) you always end up with worse quality than the original, regardless of the encoding settings. Since pretty much everything can play AAC, there's no point in converting it in the first place. Just remove --audio-format mp3, and you'll get an .m4a straight from YouTube with no conversion step.
That's right, usually the best quality audio you can get from youtube is in m4a format. The only problem I'm having is youtube sets the format inside the m4a to dash, which some stupid players (including iTunes) don't want to play. So I have to run something like
ffmpeg.exe -i "Keith Wiley - The Fermi Paradox, Self-Replicating Probes, Interstellar Transport Bandwidth-AUk6ZlePtQA.m4a" -c:a copy 2.m4a
