I wouldn't have thought to do grilled sweetbreads, I'd do something like lobster or seared diver scallops. Traditional, classic pairing, but truly divine.
The other thought would be to do a scallop appetizer and drink the wine with that, then have the sweetbreads as a main course with a nice Burgundy (Pinot).
I haven't had a Domaine Leflaive, so I can't comment on drinking window.
Good luck! Let us know what you end up doing and how it works out