For me it is certainly happy ending tinged with a bit of wisfulness, but I don't see how we could have a better one.
For all those who want BongHwan to remain in the Queens body, I like to ask you a sincere question.
Would you be willing to go back few hundred years in the wrong body and you have to have sex with someone you like, but of the opposite gender you are attracted to??
BongHwan liked the king, didn't love the king in a romantic way. that was So Young.
I think he is incomparably better off with that end rather than being stuck in a woman body.
Bonghwan suffering from the first part of the series is played for laugh, but if it was taken seriously, then his suffering is massive.
We now support (at least in some part of the world) transgender people because we acknowledge their suffering of being stuck in the wrong body, so imagine adding the time period. How can you say that Bonghwan is better off being stuck in the queens body???
People can get over heartbreaks. He will be fine in modern day.