Does iDrive have a "preference" (does it work better with one or the other, or support some features better in one or the other e.g. album art or "sort order" tags) between .mp3 or .m4a? Does it allow you to simply browse the file/folder tree as you've laid it out, or are you forced into the Apple-style Artist/Album/Song hierarchy choices?
I keep my entire library in lossless format (originally was FLAC, later transcoded to Apple Lossless so I can play it with iOS/Macs more easily), so I can spit out whatever compressed format I want as long as I devote the time to let something chug through the library. I currently have it all as pretty high-quality AAC (.m4a) simply because variable bitrate in that format tends to work better with 3rd party devices in my experience. (My older VW and Audi would refuse to show remaining track time with VBR mp3s.) I think I need to just take my stick-o-music (I use one of the tiny Lexars linked earlier - they still run pretty warm too, FWIW, but I picked the Lexar over the Samsung because it has a blinkenlight on it) and jam it into a car and play around...
I skimmed the USB PDF linked above -
incredibly impressed that
exFAT is supported.
It is technically superior to FAT32, but it generally requires a license to implement so most companies are too cheap to pay that per-device fee... Pretty cool on BMW's part here.
I assume that for voice command, it has to index the device somehow... Is it smart enough to save that index on the internal storage of the car so it doesn't have to re-index the whole stick every time the car starts again? My wife's 2017 Subaru sucks like this, making the "USB stick full of music" concept almost entirely worthless in that car (it takes 3-5 minutes to chew through ~20GB of music, every time you start the car).