The 4-stave system is interesting but can almost certainly be done using ZWJ hacks maybe.
[1] https://www.notationcentral.com/product/musglyphs/ [2] https://highperformancewebfonts.com/read/subsetting-and-liga...
There is a compositional approach, used by this font.
OP went with brute force because it's probably a heck of a lot easier up front, lol.
> Sometimes (not always), this makes addition visual
I wonder how often - my suspicion would be rarely.
Edit: There's actually a few cases with adding 2 as well!