A very similar BA/AA "moving to USA from Eu" thread on the BA forum:-
https://www.flyertalk.com/forum/brit...filiation.html
Lounge access will be a challenge as a UA ff.
With many USA frequent flyers upgrades are the primary ffp objective.
Be careful of the LH M&M 3 year hard ff miles expiry
Award cost guide:-
ORD to FRA Award Pricing Calculator | AwardAce