I figure their original plan was for Battle for Mewni (probably in a somewhat abbreviated version) to be the S2 finale, with it being something that COULD stand alone as the series finale but with the Eclipsa stuff as a hook for a possible season 3 if they were renewed.
In that event they probably would've had a Jarco breakup before Marco went to Mewni, and there'd be a Starco kiss at the end after Toffee died. But since they got renewed for a 3rd season while this stuff was still in the planning stage, they bumped what became Battle for Mewni into season 3 and made Starcrushed the finale, and maybe did a few additional fillers beyond what was originally planned of season 2. At this point they've got the idea of season 3 being the last, so they probably would've tried to stretch things out by having Marco still dating Jackie for a while. Then after the breakup and Marco moving in permanently to Mewni, an equivalent of Curse of the Blood Moon would happen in mid season 3 to throw doubt in the characters' minds about whether their feelings for each other were real, thus delaying their hookup until the finale.
But once again they got a fairly early notice of being renewed for season 4, so plans changed again. They know there's absolutely no way that with the setting moving away from Earth that Jarco would get the job done of delaying the resolution all the way to the finale. So Jackie is unceremoniously dumped from the show and Star randomly doesn't hate Tom anymore for flimsy reasons. That produces a love triangle that they can actually sustain into season 4 (even though doing so has strained credulity at times), and that leaves us where we are now.
This is all just guessing on my part, but given how the show has repeatedly teased Starco via "big emotional moments" that the characters couldn't possibly ignore going forward and then...had the characters ignore it going forward, it probably really did go something like that behind the scenes.