Really charming, strong composition on display here. Something I would recommend to help out with your mixing would be to layer an instrument that you want to really define the EQ of, assuming you don't use expensive EQ software.
Layer one with a regular EQ, how you think it should sound, how you're aiming to EQ it. Then on the second layer, run a LP or HP (Low pass / High Pass) and cut it until they bleed into each other in a balanced way. You don't need to do any fancy side-chain stuff or anything crazy. Just curve them out as if they're one composite track.
This helps create a sense of depth and texture, and the LP / HP layer also helps you pick out frequencies in the standard track that you want to close in on.
The flutes in the last track are a great example, there's some loose high end, cut out all the highs and slowly just bring them up as one filter / pass until it sounds right, then match that range in the normal layer and just very subtly blend them.
This is a great OST, really solid work.