Should I get Apple TV+ for Star City?

If your main reason is only Star City, wait. Subscribe once Apple confirms the release date and episode schedule, or when several episodes are already available, so you do not pay months early.

If you already like For All Mankind or want other Apple TV+ sci-fi/drama, Apple TV+ is more justifiable now: you can watch or rewatch For All Mankind first, since Star City is expected to be connected to that universe. But as a one-show subscription, the smart move is to delay until Star City is actually streaming.