Youtubes's best available audio is usually either 128kbps AAC-LC or 160kbps Opus -- both these codecs usually sound substantially better than MP3, and at these

Youtubes's best available audio is usually either 128kbps AAC-LC or 160kbps Opus -- both these codecs usually sound substantially better than MP3, and at these bitrates should be "transparent" with most material, (aside from whatever other processing YouTube does, or what was done before it was uploaded to them). Converting this to MP3 means a lossy-to-lossy transcode which can only cause further deterioration, and increasing the bitrate to 256kbps (or anything else) isn't going to fix that -- at best you won't be able to notice any audible difference, apart from having larger files for no benefit. Unless you need MP3 files for compatibility, instead of using the -x option to extract audio and converting to MP3, you'd probably be better off just using -f with the best audio format, and perhaps re-muxing in to a different container if required.