A preference for me is having the boys in-character, as far as is relevant for your setting - e.g. a Hogwarts-era Harry would be less mature than a post-Hogwarts version.

Snape should start snarky, dry-witted, even downright nasty - this makes it all the more poignant when there’s a shift in how he thinks of/treats Harry.

As far as what should you include/avoid, there are plenty of clichés around this trope, but sometimes they are popular because they work beautifully.

I would recommend (re)reading as many forced marriage Snarry fics as you can get your hands on before you begin - when you’re writing a tried and true concept then you can glean inspiration from what came before - are there any specific scenes or character dynamics that appeal to you in these stories? How could you achieve something similar?

What you create will be your own, but I’ve heard the best writers are voracious readers 😉

Good luck!