While it may seem like a waste, whether she was legal at 16, wasn't his student anymore etc it would still have been frowned upon by society if they got together as soon as she graduated and that's also what Miku's mom is implying as well as Nagisa a few chapters ago. It would be too fresh and I think we would see the same reactions in Western societies.
So, by waiting 2 years until she's an adult, he's trying to have the "cleanest" possible start for their relationship: she isn't his student anymore, he hasn't dated her as soon as she graduated (which could have meant that they had been together in some way while she was a student), she's about done with culinary school AND she's now a full-fledged adult in every sense of the word.
I also don't think those two years have been a waste: they've been spending a lot of time together and building up their relationship properly in the meantime, this time not as a teacher and a student, but as a man and a woman.
I also love panels like this one way too much 😍