BMW uses rubber gaskets not synthetic. Evenually they will shrink and there is nothing you can do about it but keep the seals plyable. I have had a number of BMW convertibles and each and every one will eventually leak from the front seal to the windshield from the rubber shrinking.
This can take a long time however and I think the sun beating down on the rubber accelerates that process because I have never replaced seals that were protected by the sun. I don't think keeping the top closed in the garage is going to hurt anything actually. Better to have the top down then in some popped up where the load on the hinges and hydraulics is there for however long you leave it in the garage...