Honestly it's heartbreaking to see Sorata surmise that, after everything he's done and everything been put through by every other character in this series, he's now "at the start of being a fiancé—or that's how it feels like, anyway".
At this point I have to wonder if maybe Sorata himself should stop and reflect on whether he isn't caught up in the betrothel set in place by the two grandfathers and that it hasn't influenced his feelings of love for Miyuki. Being told "you're gonna marry this person" at five years old means he likely never thought to look at anyone else and even wonder "what if?", and so this relationship with Miyuki is all he's ever even imagined, much less experienced.
Sometimes you need a second point of reference, especially when everyone around Sorata is varying levels of awful—or at best unhelpful or avoidant—to him so much of the time.
I don't think it's wrong for Sorata to end up with Miyuki, and I don't even think he'd be unhappy (pending Miyuki getting over herself of course), but if this is going to just keep going for several more volumes, an arc of introspection for Sorata as he considers his future and wants solely for his own sake, outside the context of romance, would be nice (since Miyuki apparently will likely be getting that very same opportunity).