As a Jackiefag I have to say it's incredibly clear and obvious that Star is endgame. River already likes Marco, he was there for the battle for Mewni, the whole show revolves around their relationship moving from dislike to friendship to romantic attraction while they have adventures.
I do think Nefcy will torture the viewers for every last drop of starcofag tears and character growth via pain she can, and will place untold obstacles in the path of Starco including but not limited to playful tomco romance baiting, Jackie becoming a genuine threat, Marco being incredibly reluctant to recognise and accept his feelings for Star, possibly a new possible love interest bait character for star and marco & star having some severe arguments and whatnot - but in the end, it's gonna be starco. If not explicitly canon starco then at least a korrasami type heavily hinted ending where they're both unattached and acknowledge they like each other and go away together, or something.
Manna ending would be narratively shit. Janna doesn't deserve Marco, Marco loathes her, she's not good for him etc. It'd be shit writing
at best there may be some ship baiting, but that's it