Working with a type expert to curate a list of 20 or 30 high quality open license fonts to cover the majority of use cases would be a huge step toward legitimizing Proton Docs. I can understand why you might not want to utilize GFonts but it is one of the killer features that keep people tied to GDocs. As an educated amateur I would be happy to help with this, but someone like TypeWolf ( https://www.typewolf.com/ ) would be a great choice as a consultant.