I love this answer, thanks!
I remember learning a bit about typography back in the day but it is indeed a fairly complicated (and kinda nerdy) subject.
In the past I’ve definitely added custom fonts the “old school” way, I suppose I tried the docs way becuase then it adds the font to the theme fields drop-down. Do you then remove the font option from their theme settings? I do give my clients design decisions most of the time 
This particular client is actually using an Adobe Typekit link anyway but if I just remove the font.json file it no longer loads the font (at least in the editor). In their case, I’ll probably just re-jig the thing with the @font-face rules in main.css.
I’ll have to have a think about the best way forward the next time I come across a custom font.