While he does owe the sister trio a favor, especially after one of them saved his life, part of me wishes Youta didn't make that promise to them. Since the story stresses that he is on his last life, no more fuckups, any major mistake going forward could be his last. Youta shouldn't risk his life dealing with another witch threat but instead train as long as possible to beat Kanejo.
But hey, I like that heroic aspect of Youta.
What that said, some loose ends can be tied up while he's dealing with the next witch threat. Maybe Youta can find some end-game equipment or knowledge in the sisters' village. Hopefully, in the future, Youta will communicate with the moon goddess due to one of the sisters not having a collar, and she actually explains how the world came to be.