Diesel8, I sympathise with your situation but don't sacrifice your great job. I'm sure you can get some extra time if needed.
If you joined CX and wanted FRA you would have to join CX Freighters flying the 747-200F. Not only would you have to put up with a bad roster but also, you would be stuck in HK for at least 2-3 months for initial training on the classic. As for your career, you would most likely be stuck on the classic until they retire the airframe (5-10 years or more) or you get transferred to pax (not likely).
Stay where you are and make visits!
Good Luck.