I definitely agree with you that Ainz is passive which is what makes it obvious that he’s not the male lead but that’s because of how he was written in order to make Calix stand out. In the context of the story I think it makes more sense for her to end up with Ainz because she doesn’t seem to like dealing the nobility and having to attend those banquets. She just wants to run her cafe and be separate from the gossip and rumors from high society which would be hard to do if she gets with Calix and becomes the archduchess. But it’s already set in stone so whatever.
Everyone saying Ainz should be ML and Calix is not ML material.
But in my opinion, Ainz doesn’t try like Calix to make Erin happy. He suddenly come and go that not how ML acts in manhwa.
And how is Calix acting selfish? Childish? I don't find it at all in Calix? And don't forget he is literally 5/6 years younger than Erin means he is 19/20 years, a fresh adult. It will take him time to act like a mature person (even though I think he is enough mature for his age)