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).