It’s definitely possible to find the source of your vehicle’s current drain. It isn’t rocket science. Sort of like figuring out where a swimming pool is leaking. However, the task can be quite time consuming. Did the shop explain this to you? I would expect the techs to offer the potential high cost to you as an explanation why they haven’t been able figure it out. This is even more so with the electrical complexity of modern vehicles like yours. But if you want the problem fixed and don’t care how much it costs, ask the dealership shop manager if they would start the job again, and this time keep working on it until it is done to your satisfaction, provided you agree to pay (and continue to pay) the estimated daily fees up front.
On idea: Battery acid on the battery’s surface is a possible and difficult to diagnose current drain path. An uncommon cause, but if you feel lucky, ask your shop to clean the battery case as their first step. Best of luck.