When I had my old Chevy SS with Brembos, they did the same shims and grease to address squealing. It helps for a bit then comes back. I did read about a copper paste that they apply now that's supposed to actually help a lot but have not personally tried it (different cars but maybe
this could be helpful). I think most people "fix" this problem by giving up and switching to different pads.
I have not had the squeal on my M340i yet luckily.