So, it seems like this thread died right when it was born. So let me reformulate: Read custom Json files in the data/ directory and do stuff with that. Does Minecraft have a static function to read the Json files and get the values inside that?
You need to create a literal argument (Commands.literal) for "string" and call requires/then/execute on that as necessary and do the same for "player". In your main command registration (where you call CommandDispatcher#register), create a literal argument for "testcommand" and call then on it with the result of each literal method chain.
If string and player are more like separate commands than modes of the same command, you can move each sub-command into its own class with a static method that returns an ArgumentBuilder<CommandSource, ?> (the result of the literal method chain) and call these methods in the then calls of the main command registration.
See ForgeCommand or my TestMod3Command for examples of this.
You got to remember, forge doesn't have everything replaced into their own words nor resources. Sure I agree with you in this, but it mainly tells the user(s) that it's a default Minecraft item with the assets domain.
Also, it'll take some time to replace some of it just with their own domain, but I like it.