I was faced with the same problem and implemented RawRepresentable support, as described in @Evman's answer and it worked fine for me for about a year. But then I was getting warnings/errors about adding protocol support for both a type and protocol that were outside my control. see my question on apple's dev forums: https://developer.apple.com/forums/thread/774437

I ended up creating a structure that contains my dictionary. This met my need of the being usable with the @AppStorage modifier.