Wow, that sounds great! Kaito and Len seem to give this song rather different sounds--Kaito is more soft and almost melancholic, and Len gives it a more upbeat sound. Depends on your intent, but I wouldn't give up on Kaito too quickly--he might be hard to tune, but you've done a great job with him so far, and IMO his sound fits your melody really well. Good luck with the rest of the song, whatever you decide to do!