This seems to be a common occurrence in many BMW models I know back as far as the E46, but the E46 ground strap is accessible. I often suggest to people to use a jumper cable between the engine and chassis jump start nut on a regular basis when there are no crank or slow crank conditions.

Seems it may be a good idea to just install an additional ground strap as a PM on most models. Can't be too careful and too many engine to chassis grounds.

Also assume you know a fully charged battery should have a terminal Voltage of 12.6 Volts. Your battery may not have fully been maintained with your charger while the car sat? Might want to make sure you get a 4 Amp or larger charger because smaller battery maintainers cannot actually bring a weak battery up to a full charge. Many battery maintainers default and shut down if the battery is too discharged and will not even try to charge a very discharged battery.

Where did you connect the additional ground?