Hi @rhinókerōs,
FWIW Open Sans works fine in Rhino 7 (or 8) Windows.
More specifically the variable font version downloadable from Google, showing file version 3.003. The download also includes static font versions for software that can’t handle variable fonts.
If you have the variable font versions (basically two files, one for regular fonts, the other for italics) then you could try uninstalling them and installing all the static files and see if they work better. Or if you have the static fonts, try uninstalling them and installing the variable versions.
I’m sorry but I don’t have a Mac to explore that side of the font divide myself!