Glad it's still going. Phone / message UI is one of those VN things that looks small and then eats weeks.
I can't really help on the Ren'Py side, but I built a small script-first VN runtime where multi-panel / chat append is a normal part of the workflow, not a bolt-on. Not suggesting you rewrite Mono no Aware mid-project. Only if you're curious for a next game, or want to see how another tool handles that pain. Happy to help with the tech side if you try it.
https://stopandgo.itch.io/vn-mikro
On music: I used free piano tracks on my jam VN (also my first game) Cold Down: https://stopandgo.itch.io/cold-down
This artist has strong free-for-noncommercial piano stuff:
https://pixabay.com/ja/users/harumachimusic-13470593/
https://audiostock.net/artists/3556/music
Anyway, still worth double-checking the license for your case before you lock it in.