Any concerns adding user role ""everyone on the internet"" in an existing application?

I would caution against having just some tables open to EOTI.  If you set up the proper permissions, there is not problem.  But the risk is that you or your successor makes a mistake in the future and accidentally opens up that Role to see tables they should not.

So it's a judgment of risk of a future error vs, the extra work of a separate app.  If you can solve your issue with a cross app relationships, I would just do that.