Need a few more pieces of info.

Are you flying/paying for economy (discount), economy (full fare) or premuim cabin?
If economy, is "free" Economy Plus access important? or would lounge access be more important?

Seems your primary goal is award flights and upgrades and your paid flights will be mostly on UA. Correct?

Is elite status a goal? You are expecting to do 20,000 flight miles of annually? Any chance you would be over 25,000 annually? What will be you annual spend?

Generally it is best to use the program of the airline you fly the most.

Aegean's (A3)program was a trick to get lounge access but A3 has tighten up their qualification.

Lufthansa's (LH) program will not make much sense unless you fly premium cabin on one of LH carriers.