Calming instrumental~ 
I remember watching this a while back, looked through some clips of it and this one hit real nice.

Memo Sepia / Gray

Mob Psycho 100 S2 - "Gray" by sajou no hana [Instrumental]

Youtube video: https://youtu.be/0tMvzvVXnpo

Try to hear this song! This is true full ending of Re:Zero !