Honestly? I don't ship Starco but think it'll be the endgame ship, they've focused so much time on building their relationship and dynamic I somewhat doubt they'll actually go the "accepting that sometimes you're better off as just friends" route, even though that'd be a baller narrative move. But considering it's a kid's show, I don't think the creators will have the guts to not make starco canon in the end.
However, "will they or won't they" is a time-tested great way to build romantic tension and get fans invested and keep a dynamic interesting, so bringing the couple together too soon would be a bad move storytelling-wise. It's much more difficult to keep an established couple interesting than it is to think up ways for them to keep chocking on the confession and feel distraught and hesitant and hopeful and grow slowly closer but never quite spit it out etc.
Also, Star really has a lot of character flaws, and needs to grow and develop in order to "earn" Marco imo. She's been quite a spoiled, lazy, selfish, stubborn, violent brat so far, and I feel she has to to through some emotional pain and growth in order to become a better person and learn to take the needs of others into consideration etc so that she can become gf material.
So no, in Just Friends I don't think Marco will choose Star over Jackie. The story isn't at that point yet.