The solution to all your Cap related problems... Spoiler : Peggy Carter DID marry one of the rescued soldiers, and had kids. Returning the stones wasn't a one day (of Cap time) event. That's a career, not a lark. And he's already lived...what...fifteen years since he came off the ice? So he wouldn't go back to 1944 and show up twenty years older...he'd go back to a time when Peggy had already been widowed or divorced, with both being likely outcomes for people in her and presumably her husband's profession. Which is also, generally speaking, a profession for the young. With a loss of husband and a couple kids to consider retirement from SHIELD and associated dropping off the radar wouldn't be unexpected, and would offer a perfect time point for the older Steve Rogers to insert himself...and enjoy a quiet lifetime until he safely watched himself return from the ice and carry Peggy's coffin as a younger man. Then he makes one last time jump to meet up with Falcon, the end. And, yeah, the Thanos thing is a total game breaker.