Sorry. But this really, really sucks. I just don't get why they made it so impossible to get any good sound into this.
They didn't make it impossible- they just only offer one method.
Put uncompressed high quality audio on a USB stick.
I've got 250 or so gigs worth in my Tesla. Plays beautifully.
For a brief while I put into my minivan my DIY desktop speakers with a small SMSL amp and AQ Dragonfly DAC. I could do that again but the M3 is a lot smaller than a minivan and it would take up some room. It wasn't permanently installed though so I could just try it. The one thing is the power requirements of the small amp was met with a plug in 12vdc to 120vac inverter in the car. I'm not sure the Tesla could handle it. I guess I have a task for this week to give it a try.
The 12v in the center console is rated for 16 amps peak, 12 amps continuous FYI- though you'd want to possibly look into wiring into the penthouse area for a more permeant install as many who've added their own car stereo amps and subs have done.