[DonVoigt]

Peening the flange or a shim- as you say.
A shim that thin will be a bear to make, you will likely need a surface grinder and magnetic table.

With either method, I would add loctite to the threads on final assembly; after confirming headspace is in spec.

If this is not one of the matching pistols, you may want to consider a different barrel, that one does not have the greatest bore anyway; of course that would mean having several barrels to choose from for fit.

Lugerdoc or GT could help you with a different barrel.