tbh I'm a fan of a very lowkey, "realistic" proposal for them. Like they're both in their late 20s (probably early 30s depending on when this happens), already have a long history, tired of dancing around that line dividing whatever they were post-POP to this new established third thing so I feel like one day Alhaitham would just casually mentioning it during dinner at home. Something like, "I'd like to have a life like this with you for the rest of my life, would you like that?" :'D