Does the resources plugin also allow defining the data objects returned in a way that would be shareable between the client and the server (to fully specify the API in a central file)?

The last time I checked was just after 2.0 and it was not possible. Looking at the documentation, it doesn't seem to have changed, but maybe someone else can confirm or deny. In the end, I created my own wrapper for it 😕

So it is a valid use-case for it, right? I was a little bit worried I might be trying to use it wrong.

Well, all I can say is I had the same question and did not find an answer, maybe there is a way I didn't see either?

There is no builtin mapping of resources and input/outputs. But if you want to map it manually, just define the data transfer objects in common main.