Beautiful. I can't think that I'd change anything. Your cello naiveté may in fact be of benefit here, as you've created a simple but very opulent pad that's geougeous with the singing voices and piano.
Having said this, if you contemplate a whole album in this style you'll need to work to introduce enough variation between pieces to hold interest with this limited, but beautiful, palette.
The Sufjan Stevens approach might give you some ideas in that regard.