After a few days I've come to the following conclusion:
There is a definite drop-off in sound quality between Bluetooth and AUX. AUX quality is not satisfactory to me. Therefore I prefer the OEM Bluetooth configuration as described
here.
I have the phone paired with both OEM (voice & audio) and Andream screen bluetooth. Of course only one BT connection can be active at any given time. I'm not sure which it is and there is also a WIFI connection in the mix that I'm not sure what it is doing or not. But I dont really care as long as it works which it does.
So I have OEM BT audio/music and I have OEM BT voice calls both passive & active. Meaning I have good sound quality and I have good phone call quality. And I have CarPlay with sounds/music apps and google maps. The Andream screen appears to act mostly just as a screen/control unit. Not quite sure yet what difference having AUX configured as 'manual' over 'auto' makes.
Back and forth between iDrive and Linux screen works perfectly. iDrive is a bit stretched on the 8" screen, but I dont even notice it anymore after a few days. This looks normal to me already. Both centre console and steering wheel controls work for everything I do both for iDrive and Andream screen navigation also for volume, answer calls etc
I havent gotten down into the finer details yet. As in mixed music and voice during google nav operations. Or Siri. Not a Siri fan anyway.
Overall this is pretty nice functionally and it looks really great. Dont get any glare from the screen. Love the reverse camera.
The hardware is good.
The Andream screen's USB cable I leave behind the easily removable trim under the head unit. I will use this just for boot logo or other updates meaning hardly at all. On the OEM USB in the armrest I use a short USB extension lead with charge/only, data pins not connected. Means when I plug the phone in for charging it wont mess up the entire connection situation by trying to go to OEM USB.
However, the unit seems to have its 'moments', these seem to be down to software. Once it started up just with a black screen but sound was working. Ignition off then back on and it was back to normal. Once it seemed to need a minute or so until it had itself all reconnected.
I'll wait and see how this pans out in the long run. If this remains somewhat stable I'll be very happy with my purchase.
Edit: After two weeks this is pretty stable. Still reconnection every morning on automatic. The switchover to OEM UI etc is not ideal when making/receiving calls but it's seamless and it works. The whole thing isnt perfect perfect and you feel it could be if only they worked the software a little over but it's very good for the money and I'm very happy.
The alternative is full NTB 6 upgrade with hardware that costs 1.5k (second hand) and vin coding necessary etc. This is a very sensible upgrade and it's good enough for me.