I found the final battle a little disjointed and hard to follow with all the cameos.....felt a little forced, IMO. Minor nitpick.
I'm not at all a fan of comics, so the Captain America thing didn't bother me. And I wasn't looking to dissect the time travel. But I get it - that happens in A LOT of tv and movies where they are going for a certain outcome and are kind of ham-handed in executing it.
It's theoretically impossible for everyone to think you did everything well. And the better job you do, the more people you set-up for disappointment. "Fan service" is probably a term that's thrown around far too liberally and loosely.