If this is your first go at this type of thing, you did really well. The last track shows the fuller potential of this OST. I think the more minimal arrangement works fine for these pieces, but I also felt at times that I was hoping for a bit more texture, color, and thematic material. The soundtrack works well as is because there seemed to be a unified style to the compositions, whether that was fully intentional or not. Seeing as this is your first time, that is totally fine (and would be if it weren't your first time also). I would encourage you to explore other ways of creating sections in your future pieces. This can help melodies take different shapes and arcs.