what if Starco doesn't happen for the entirety of the series apart from one episode set in the future that shows Star and Marco as king and queen? with their kid, perhaps.
so Marco tells Star that he doesn't feel the same way for now but reassures her that things could change (last ep of season 3, maybe). so he still spends the rest of the series dating Jackie, but we also get to see that things do change in the future but it doesn't happen instantly.
i'm not sure anyone would like to see this cause it sounds like a bit of a cop-out, but it'd at least help the writers avoid doing an asspull regarding Marco realising that he likes Star all long despite little evidence.
maybe show Jackie & Marco's relationship deteriorating throughout season 4 (assuming it's the final season) until they break up at the penultimate episode and then throw in the episode set in the future in the very next episode.