Here's the problem with BMW cooling systems. They went to 'environmentally friendly' materials in the late 1990's bragging in their literature that the entire car was very ecologically friendly when used up and scrapped. The trouble then became that the plastics on the water pump, radiator and coolant tanks decided to re-cycle themselves much earlier. They got rock hard with heat and the vibration cracks the plastics and you overheat. It's a known issue on all BMW's and happens around the 100K mark. So what those of us that pull our own wrenches do is replace them with aluminum aftermarket parts - especially the radiator - and that solves the issue. More costly than the plastic OEM parts, but no more crapped out coolant systems.
Did you know that if you join BMWCCA and hold your membership for one year, you get very nice discounts on new and CPO BMW's? Here's the current structure:
https://www.bmwcca.org/vehicle_rebate
Also there is one for BMW motorcycles as a CCA member, and that one is instant.