> In other words, there is still a (lossy) translation layer, it just happens to be in the RDBMS rather than in-app.

It's not lossy if your application can guarantee a json <-> datatype roundtrip and the json is validated with jsonschema (generated by your application)

In Rust it's something like this

https://serde.rs/ to do the data type <-> json mapping

https://docs.rs/schemars/latest/schemars/ to generate jsonschema from your types

https://github.com/supabase/pg_jsonschema to validate jsonschema in your database (postgres). with this setup it's interesting (but not required) to also use https://docs.rs/jsonschema/latest/jsonschema/ to validate the schema in your application