Sorry to bring the bad news but from my personal experience the only real fix is a transmission replacement (BMW-covered in my case, twice, but I had to go to another dealer) and possibly also a clutch/dual-mass flywheel replacement (got ripped off on that one to the tune of $4800).
Your synchro grind will only get worse until you cannot shift any more. You can only delay the inevitable, which your dealer may try to do by proposing half-measures. Hopefully your dealer won't do that and instead give you a new transmission, good luck.
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