Well well, it looks like this issue has finally caused a complete failure. Car just cranks (not the starter, not the battery) so it seems the transfer pump/fpr has finally died.

Has anyone had experience with DIY'ing the replacement of this part? Looks kind of complex.

https://www.realoem.com/bmw/enUS/sho...diagId=16_0659

Part 2 on this diagram (16117195469).

I also came across this tonight, which can give a no-start condition. Not sure if it has been done on my vehicle, but will try to ask BMW next week.

https://static.nhtsa.gov/odi/rcl/201...6V746-3479.pdf

Update: the car is running and driving again. It seems to start fine in the morning after sitting all night.

I'm calling this a confirmed fix. The car starts right up every time (so far) and sounds / drives great. Let's hope the issue doesn't come back.

Complete DIY here: https://youtu.be/7N78Y_5Mcz8