Put your charger on the battery overnight.
Sounds like a 2-6 Amp charger is fine. Put it on 6 Amps, if it is an older school charger with the Amp meter, most start out around 3-4 Amps and as the battery comes up to fully charged it drops down around 1 Amp or less.
Sounds like the type of driving is probably not great on the battery charge status. I usually put the charger on a lot of my vehicles about once a month as a maintenance process. Not sure you need to keep the battery on charge all the time, but monthly overnight is not a bad idea.
The problem with the parts stores testing the battery, they use capacitive/resistive testers which are pretty good, but the people using the testers often do not bother to tell you the battery is "good", but needs a good charge. They just tell you the battery is "good".
Since the vehicle sat for a few weeks, if the battery was low to begin with, the starter draw may drop the cranking Voltage to the point that the iDrive drops out and has a hard time recovering.
You do not need a very complex multi meter, however, this is a decent meter at a good price at the moment with free shipping. It has Max & Min hold which is very helpful can can be used for the car or around the house -
Power Probe (PPRPPDMM)