I just did a buy back with a 2017 buick regal GS that had less issues than you have experienced. I just kept taking it back and calling GM until they gave me a case number. The dealer I kept taking it too finally told me they would not work on it any more. When all was said and done..I showed them that they had done 19k worth of service on a car that's olny worth 19k.....and since they never satisfactorily fixed the problem that I would keep coming ...That was the ticket....They refunded my entire amountplus all the payments I had made and subtracted a small amount for milage.
Now Im driving a BMW and am happy as a clam in sand....Just keep taking it in and don't get discourged