I would recommend Security Aviation at HHR. They took care of my 520 for a few years and Glenn's 680FP when Moe owned that. They took care of his for decades. They know geared engines. Ken is the mechanic there who knows the Commanders specifically. Great outfit, honest people that do good work. Talk to owner Chris and say I sent you.
If that doesn't work, Imperial Valley Aviation at Chino has Commander experience. Robin Howard at Howard Aviation at Brackett Field is mainly an avionics shop, but they've taken care of a 680 with Mr RPM conversion for many years as well. Both are good.