Code is itself data, so a full decoupling is logically impossible.
Data is going to have an implicit schema regardless because that is just how data works. And once there is a schema, it may as well be expressed explicitly independently of the code because then you get the whole basket of standard schema operations for free (validate the data against a schema, provide a schema to an external consumer when moving data around, talking/operating more generally on schema to manipulate data, generating glue code or APIs programmatically).
Your description sounds like you are using your objects as schema references which is fine, but if there is a 1:1 correspondence with schema then you are already doing data oriented programming, and if there isn't then you can't have 3rd party libraries that support schema-based operations. And losing those schema-based operations hasn't gained anything because the data still has a schema, it just isn't well organised.
TLDR; Data oriented programming isn't essential. But if you plan on passing data around between systems schema should be mandatory, and if you pass data around within a system schema are recommended.
> Why would I write a separate schema when the object itself knows what it will accept, what is optional, and what range the values should be in?
In practice, I have seen a fair number of complex objects where that information is obscure. If there isn't an explicit schema there is a chance of bugs where the object doesn't understand the data it is ingesting and that it won't share its knowledge that there is a problem until it fails in some obscure way in runtime. It wastes a lot of time fixing those bugs because the easiest way to clean that up is to tease out an explicit schema & start thoroughly validating inputs.