I think I found it, I got the default font index wrong (starts at 0, I thought it started at one), it's working, thanks! The thing not updating was because I was looking at the wrong font, as above...
I also noticed it doesn't override system colors (the kinds you can change inside YEP CoreEngine) but that may be a feature, idk. Anything more specific like changing HP MP TP font would need a purpose-built plugin anyway :3