I often find piano songs very hit or miss, as I'm usually left to want more in terms of instrumentation.
I think in your case, the limits actually contribute to the charm of the song as it lets the AMAZING composition work shine through, it really adds to the emotion.
I have one single issue with the track, and as you can probably guess, it's the vocals.
They sound truly beautiful (I'm vibing so much rn), but I feel like they are introduced too suddenly and loudly (which caught me offguard).
I believe adding a slight reverse reverb tail before introducing them would fix the issue for me.
I also think that the vocals peak a bit too high in volume in some places, and that a little delay here and there at the end of a section could make it even better (though this is personal taste).
Very happy with this !