Thanks for the reply!
I was really hoping I could use the automatic synthesis to easily and quickly (and basically freely) implement game save/restore, but my game state is big and complicated and has a lot of references between things. It's possible I'm thinking about all of this all wrong, though, (including the idea of representing my save-game this way) but it seems like giving up the automatic synthesis would be more work in this case than implementing a custom encoder/decoder for a simple made up file format or something. Looking through the JSONEncoder/JSONDecoder code you linked, it seems to get pretty complicated, unfortunately.
While searching around, I ran across this project which might be the simplest reference point for implementing an Encoder and Decoder, but I haven't dug into it yet: GitHub - Flight-School/Codable-DIY-Kit: A template for creating your own Swift Codable encoders and decoders