How much do I need to know about Ian before All-New Cap? I'm super interested in Steve having a kid, but I never finished Dimension Z because it was a little too apocalyptic wastelandy for my taste. Also, are Steve and Sharon still together?

I think all that you should need to know is that he was raised by Steve in Dimension Z, and that now he’s Nomad. Apart from that, you ought to be able to get right into the story.