Plane2323....... I am doing the same VCE-ORD route in about a month, the identical situation trying to move from LX to LH. Interesting enough, but I don't recall there being LH F-class on the MUC-ORD route. I believe that route flies an A350, and I believe they only have Bus on that route for that plane.
If you have never experienced the FCT at FRA... it is special for the one time experience, not to mention the bragging rights when you get back home. The ride from FCT to your plane is kind of cool, but getting out and being so close to a jumbo plane is incredible. Sometimes you get some envious looks from other pax when you step out of the elevator that take you up to the boarding bridge and they usher you ahead of everyone else.
4h in FCT would be a pretty ideal amount of time there.... have a wonderful brunch between flights with a cocktail or two... and not be rushed at all.
Doing FCT in FRA and then doing FCL in MUC would be a great way to experience both.... I think you will find both wonderful experiences.... and the car transfer is the only benefit....... in my case, Mrs. B would not be happy with either the early flight departure out of VCE nor the extra time and agony of making two connections on a return flight after 2 weeks of traipsing(sp?) around Europe.