I'm much in the same boat as the OP, I have about 6.5hrs in FRA coming in from IAD on UA and then leaving on Aegean airlines on a Thursday evening.

How reliable are the ICE trains? I like the idea of taking the train out to CGN for a few hours and back as a good use of the time.

Can anyone advise about how much time to expect in FRA immigration/customs around noon on a Thursday (I have 17A on a 777 so towards the front), and then clearing security/checking in/etc to a FRA-ATH LH codeshare around 4-5PM (departing 6PM)?

edit - wrong days of the week