Well this took its sweet time, but the build is finally done and the car is delivered to its owner Been working on this off and on between the projects on my main job.
Changed the setup a little bit in the end. The owner wanted to retain the BMW I-drive functionality (even though this does not do much when you take away the audio control settings), and the idrive screen/control wheel is controlled by the OEM head unit. So we agreed on ditching the Halo9 install, keep the idrive, and use a phone/dap/tablet etc as the audio source, since she 99% is using spotify for playback anyway.
And then the most convenient way was to use a Helix bluetooth streaming module. But since the M Four DSP amp does not (yet) have an option to fit such a module, I swapped it out for a regular M Four, plus a Helix DSP Mini with HEC BT module. This setup only use 5 channels anyway, so no need for the 10 dsp channels in a M Four DSP.
I installed a Helix URC.3 controller on the side of the centre console, to control both main and subwoofer volume.
I do have some build pics from the trunk build, but at first here are some pics of the finished trunk: