About the youtube playlist to mp3
Audio-only output is the right choice whenever the picture carries nothing. A one-hour talk is roughly 500 MB as 720p video and roughly 55 MB as a 128 kbps MP3, and the listening experience is identical on headphones. For commutes, runs, kitchens and cars, MP3 is simply the correct format.
Bitrate should follow the content. Spoken word — interviews, lectures, podcasts — sounds complete at 96 to 128 kbps, and mono halves the size again with no perceptual loss. Music deserves 256 to 320 kbps, or M4A/AAC at a similar size if your player handles it, because cymbals and reverb tails are where low bitrates fall apart first.
Metadata is what separates a library from a folder. Keep the playlist index in the filename so albums and lecture series stay in order, and set the artist and album fields once for the whole batch rather than per file. If your player reads folder.jpg, drop the cover art in the folder and every track inherits it.
YouTube Music playlists usually carry cleaner metadata than the same album on plain YouTube — real artist names, real track titles, fewer 'Official Video (HD) (Lyrics)' suffixes. When both versions of a playlist exist, start from the Music one and you will spend far less time renaming afterwards.
One honest caveat: converting from a compressed video source is a second lossy step. It is inaudible for speech and fine for casual listening, but it is not an archival master. If you need lossless, buy the release — a downloader cannot recover detail the upload never contained.