Glad to hear you got it sorted Rolf.
It's another example of little PITA things that should never have needed doing had it been done properly at the factory.
But it is what it is.

FWIW, if I was opening up the pickup hole in the bridge by only .5mm x 2 , I would have done it with a combination of a flat file for the sides and a round file on the radiused ends. It would be much slower going, but it may have been gentler on the plating (or maybe not given how thin it is). Sometimes even a dremel on it lowest setting can be too aggressive for some jobs. Also, from what I've found, the finest grit with those sanding drums is still quite coarse.

At the end of the day you got it done without further expense. That's all that matters.