https://en.wikipedia.org/wiki/Variable_bitrate
You can spend the next 100 years reading internet discussions of "which is best for sound quality". And most of the stuff you'll find is complete nonsense wrapped in fancy psuedoscience discussion.
What I'd tell my music loving friends that care about sound quality and want flexibility: I'd suggest VBR and choose whatever quality gives you estimated bitrate of 256kbps. This is more than enough potential quality and probably overkill. But 320kbps is major overkill. And keep in mind that for the *potential* best quality, you have the FLAC lossless files. That said, the odds of being able to tell the difference between the FLAC verison and a 256kbps version is essentially zero. But it's nice to have the LOSSLESS version (like FLAC) as a permanent archive (as you can then create any sort of lossy versions later if you choose to; say some super duper new lossy CODEC gets invented that can save 1000 files in the space of 10, while maintaining good sound quality. In this case, you have the FLAC files and with a few mouse clicks can create an archive using the new codec, with no re-ripping needed.
And since you have the FLAC archive version, why not use that at home in your Roon or Sonos setup. Then you don't even have to question "best bit rate".for stuff you're using at home. Stuff you're using on the move, is coming from Apple and more than good enough for listening on portable devices.
p.s. I used to be a major iTunes user and I understand where you are coming from trying to maintain this. I tried for several years. But the best thing I ever did was give up iTunes all together. I now don't have to worry about maintaining an iTunes version of things. I only need my FLAC files. These work in my Roon setup, my LMS setup, on my office computer running foobar2000. And I use TuneFusion to send files to my iphone and ipad for use on my portable devices WITHOUT THE NEED FOR MAINTAINING A LOSSY (MP3 OR AAC) LIBRARY.