i didn't think it was abrupt at all. but then that's my opinion. when i read the first chapter the prologue just felt like nino was thinking about his life outside the walls as he was going to meet subaru to give him the money for his family. then you tie the prologue and the first chapter together so nicely by repeating that scene with ohno and nino just when they are older. Not giving the backstory keeps me interested and wanting to know more. i like trying to guess what happened.

i think it's fitting too cuz if you think about it, when nino met ohno as a child, that marked a change in his life because you mention that it's the last time he climbed over the wall. then now when nino meets ohno again, obviously something is going to change in there relationship otherwise you wouldn't have a story.