One other suggestion, if you like to stick with StarAlliance, may be you should consider other members like TG to accumulate miles. Reason being, you will be able have lounge access after getting 50k miles (*G) in that program and it will last for 2 years. If you use United or US to accumulate, there will be no lounge access if you are not flying international. Since you are planning for more than 10 trips (10x18k=180k miles), I would suggest getting the first 50k on TG and spreading the balance on UA to accumulate to their elite level of 1k so you can get 6 free System Wide Upgrades (SWU).

OTOH if you are not interested in domestic (US) lounge access, the best would be for UA, as after the initial 25k, you will get 25% bonus miles (only for UA metal) and after the first 50k you will get 100% bonus miles on all UA flights. That is a lot of miles even if you take UA from DEN-FRA or DEN-MUC.

Happy flying, and welcome to FT