You should first of all add the desktop plug to your apps section (that gives your snap access to fontconfig) and since your snap requires GTK3 i’d suggest to also use one of the gnome extensions, that will make sure you have a default set of fonts, themes and settings everywhere … IIRC it also helps with setting the proper locale of your snap …
There is no need for your snap to ship ubuntu-restricted-extras at all for the problems described in the github issue, just make sure the integration with the host works correctly instead …