open with pads made from piano echoes, some reverb. one high and one low (independent) and maybe one mid, but that should be quieter to leave space for main piano. 1 pad for each chord. Cm/Fm.
soft rain just above mic hiss, you could hear if you're listening carefully
performance track comes in, center and loudest, piano and vox.
a few verses, as arrhythmic as nec., with pads lined up to match chords, rain still in bg, and other section between verses. 3 maybe 4 verses (short).
pads enrich and embellish, not draw attention (made from echoes). low/no attack (smooth). use shadows of night as study of how to use smooth pad in a track.
performances goes to end loop, slows to final chord/note. rain a little louder. pads a little louder too?
optional: texture for end Cm/Fm combined, perhaps echoes + layered loop (backward, stretched, modified in some way). more attack ok. keep light and thin. plenty of other stuff going on and don't want to overwhelm.
counter argument: do I want to have anything in the track that sounds unnatural? would it subtract from the overall environment? perhaps err on the side of keeping it quieter, like a rich texture that you can hear when the main piano is quieter and you are listening closely. that way it can enrich without distracting.