A year ago, I bought a Nexus 7 tablet, did the OTG charging setup with a USB hub, connecting a USB DAC, OBD2 adapter, USB storage, and camera input (for rear camera). In the double din cavity behind it, I squeezed a 50x4 amp and a 500x1 subwoofer amp. It was a SUPER tight fit, but it worked.
The biggest challenges I still have: the "Timur" kernel doesn't always put the tablet to sleep properly when off charging. Also, charging in general doesn't seem to keep up, as the battery will usually die (after 2 different chargers). Finally, each time the tablet reconnects, the USB storage sometimes takes several minutes and many attempts of connection before it would read and allow music to play.
Before this, I tried a cheap double-din Android touch screen head unit. I'm tempted to buy a newer, higher end version of one of these, but I still don't think it will be good enough. Maybe I'm too picky...
Many others with my same setup reports success, so I'm sure a little more work would get it going. When it works, it's really awesome. Otherwise, I'd say dig through some of the pre-builts, like Pumpkin or Joying. My prior one was Eonon and was just very slow and unresponsive. Also, don't bother with the "physical" volume knob on these... they are simply a digital controller in the android OS.
Here's what I would get if I were to buy one:
https://www.amazon.com/dp/B07MQD4BP2...v_ov_lig_dp_it (and I will probably replace my setup with this, removing my 50x4 amp and relocating the 500x1 amp)