Thank you for taking the time to leave a comment and for liking the game ! <333
"For now", Ending C is considered the best outcome. This is because Yuki isn't actually threatened by the mc being friendly to the Oaksons. He himself likes the old couple, and so would just wait for another opportunity to meet with the mc formally.
As a little developer note, I'm considering adding another branch from that point in the future that could connect to later chapters. For now, though, I'm focusing on the main storyline first, and that's why in the ending card, you have this little mention of more to come, with "Somehow you ended up making the best choices. Till now."
Anyways, thanks again <3