After spending the last few days reading everything I could find, I think I need to go with something OEM-ish. A Renesas R-car D3 seems to be a good choice if I can find one, or a similar purpose-built computer. I can run the gauges and integrated MMI on the cluster screen, and also drive a large TFT screen in the center console, eliminating the head unit. Hell, there might even be a way to ditch the HVAC control panel and integrate that.