The SEL with the upgraded Sony system and MFT has the most audio input types I've ever seen on a production car - the only one you haven't mentioned is the composite A/V Line in which is also inside the storage for the center armrest.
My USB stick was never indexed more than once for each device, are you sure you are using a FAT32 formatted stick? Admittedly I haven't added new content to the USB in a while.
The Bluetooth client implementation in the audio playback app you are using can make a serious difference. Bluetooth playback in Spotify was horrible - it would stutter or track commands would cease to function, the audio would switch from stereo to mono for no reason, Google Play Music worked a lot better. Model of Phone also makes a difference, if you have a phone with a small amount of RAM (e.g. 512MB) for example it can have trouble with playback sometimes.
SD Card is usually used by the GPS for Nav data so that's probably out.
The only other inputs I know about in general that the car does not have are optical or HDMI and neither of those would seem to get you any better improvement over the others in terms of what you'd normally be doing with it.
Maybe you can find a 3.5" jack at 90 degrees or something? When you close the center armrest you are routing the cable through the little holes so it doesn't get pinched?
Is it a CD drive or a DVD drive? Probably CD. I just wonder because of the A/V composite inputs. I never tried this but if it's a DVD you could store significantly more data.
Edited by jestevens, 21 September 2016 - 08:36 AM.