And they're eight years apart. It’s actually quite tamed in my opinion
Doing maths you can find that Ilya was 16 and dude was eight and then you add that of 10 years they did not see each other which makes Ilya 26 and dude 18.
I got my timeline from this: https://www.mangago.zone/read-manga/kouguu_no_omega/uu/br_chapter-194172/pg-1/
It states Ilya 18-26. It's also stated Khalid is 10 when they get married and he's 18 at the end, thus 8 year time skip. 18+8= 26, so Ilya is 18 at the time of the marriage and they have an 8 year age gap.
But regardless, it stands that their marriage was political with platonic feelings at the beginning.
at some parts, it felt like ilya felt romantic attraction to him even before the timeskip which rubbed me the wrong way. without that, it would’ve been stronger imo
It is made clear in the beginning that Ilya is way older than Khalid, mostly because it is a political marriage. Ilya was the only Omega of his country's royal family, unwanted mostly, and Khalid was the young king in a country that values omegas. Although Ilya found Khalid interesting, it wasn't because of romantic love, but because he has never been treated like an equal by others. And it seems Khalid developed some sort of puppy love for Ilya, however that was one sided.
Over time the two began to care about each other, but it wasn't like lovers but just two people caught in political drama. It wasn't until years later that both actually developed a romantic relationship based on the more platonic feelings from the past and how they valued each other in the present. They didn't even act lovey-dovey until the Civil War ended. So yeah you can be put off by the political marriage between an 18 year old and 10 year old, but there wasn't anything romantic or sexual until 8 years later (in which both were apart due to a civil war).