I would have assumed garlic was in the duxelles anyway! - and I don’t think you’d need more than the usual amount of onion/shallot in the duxelles. I’d reckon the duxelles is going to have plenty of flavour, so adding more to the patty isn’t going to really do much. Just some salt, pepper, and a bit of parsely?
I haven’t tried anything like this before (only ever made duxelles for a Wellington), I’d be interested to hear how it goes!