As said, I never had any problem with the BB or crankset of my Birdy 3 and never dismounted it.

And I do not know the specification nor the brand of the crankset and external bearings mounted by Riese & Müller on the Birdy 3, there is no type or brand name on mine, only Birdy written on it.

In case of a square taper, there are several axle length and its the combination of the axle length and crankset that defines the chain line.

You need to have a chain line that fits with the rear hub and cassette you will install instead of your current Nexus IGH.