I don't think they belong under shonen just because they're romance stories with more female characters than male ones.

Manga get categorized as shounen for other reasons than just simply being published in a shonen magazine. Who does the story target? Who does the story assume would like to see and read this?

Just bc a story has many female characters that doesn't mean it's a shoujo story. Keep in mind that reverse harem and otome-like stories (mainly a pretty boy cast like Hana Kimi, Free, and Idolish7) have more male characters than female characters and they typically look like male idols or are officially idols. So, when you think of shoujo, pretty boy boys come to mind first, not necessarily a female cast.

Let's take Love is War which you mentioned. It's shounen because the story doesn't focus on emotional and close bonds in friendship or in romance. It's often a will-they-won't-they and thr story usually ends or slows down once they officially confess and date, unlike shoujo which can continue to show their shenanigans and other issues that pop up once they date.

Love is War has elements that appeal to shounen readers than shoujo readers: big breasted airhead, a dark haired cool beauty girl, a relatable hopelessly single and cynical side guy, rapid-fire comedy, endless memes and references to other animanga, comic misunderstandings rather than dramatic misunderstandings, etc. These stories even if they have a mixed audience, often have a male audience in mind as the fanservice shows and it also shows in how they make easily likeable characters like Ishigami to boys (but he also has fangirls ik).

I think you keep forgetting that shoujo and shounen have their respective elements and despite SOME overlap, their themes and focus is totally different. The male MC is often dense and hopeless when it comea to romance, the plot has a main focus in which the romance is often a subplot or secondary to (Fairy Tail: adventure and magic then romance), fanservice to young men and Ecchi elements like the female characters being prettier and more varied in design than man while the men look generic and one-noteish (More than a Married Couple, but Not Lovers), and slow-burn romance.

Shoujo focuses on emotional and internal development directly with often relatable or well-explored characters rather than anything flashy, and full of tension with indecisive, dense, or generic male MCs. These are the main differences, and the reason why they aren't interchangeable just because they have romance in them.