Iconic sells two different length peg drive bolt for the newer BDRs. The longer ones are typically use on the front hubs as they have a bolt on pin drive adapter the rear adapters are integral to the hub and the bolts can be shorter.
The long one is 6.5mm (.25" longer) probably not enough for your needs but if you do the measuring of the hubs and rotors you may be able to use a thicker spacer than 1/4" once you see what your thread engagement really is. Spacers should be a 5x120mm pattern (BMW)
https://www.iconicparts.com/peg-drive-bolts.html
If you have a 4 wheel thrust alignment done they can tell you if its a Drivetrain shift or not. If its not the Drivetrain that pretty much only leaves the body sitting on the frame cockeyed.