It sounds like you need to find a good BMW independent specialist or auto electrician to carry out some diagnosis.
You are correct that just plugging in a diagnostic computer is only half the solution. You need someone who actually knows how to use it, rather than just scan and clear the codes like some so called "experts" do.
It could take a few hours of hunting to pinpoint the fault(s) so be prepared for a bit of a bill. It's a good idea to find a specialist who knows the car and it's systems as they'll have a good knowledge of what can be common failings and probably find them quicker than a regular garage can.
All the best of luck with it.
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