I would still check back pressure to be sure the other cat is not starting to fail and the new one shows back pressure in the acceptable range.
If any of the coils (5,6 or 8) were actually bad that could be the source cause of the cat failure as that would cause unburned fuel to accelerate cat failure. If you have the old coils they would be easy to test.
The DME reads input. One of the sources for the input are the Oxogen sensors. O2 sensors are a maintenance replacement part at 100,000 miles. It is not unusual for a sensor to fail earlier. The O2 sensor could be cause of rich condition.
Should it trace back to the DME, I found the following source that repairs DMEs. No idea if they are good or bad but maybe someone here knows.
BMW DME ECU Repair