So I have enable aptx and aptx hd on my rooted Nexus 5x running Android 8 dp4
I have pixel theme installed through substartum and I have changed my device model to Pixel (by editing build.prop) am not sure if it works because I don't have an audio Bluetooth device.
I am new to this community so I don't know much about zips and flashing them but for the aptx and aptx hd to work you probably need to flash an aptx and aptx hd zip file
If it works for you please tell me
Hope this helps