I've taken to using an intermediate 'service' layer (in the parlance, but not structure, of overholt) to manage metadata on fields and inform serialization. It looks a bit like Marshmallow, but baked in as a Flask extension.

I've been kicking around different approaches to this for over a decade, going back to Castor (Java data binding), and with Python, SQLAlchemy, and the libraries mentioned here I'm finally feeling optimistic.

Request parsing in Flask-RESTful seems bolted on, and the fields in WTForms seem frustratingly redundant given that almost everything I do is going through the API anyway. Maybe focusing more on the Fields themselves will help tie it all together.

Flask-RESTful worked out for me, I like how easy it is to define routes. I'm eager to try Falcon next: http://falconframework.org/

I tried both flask-restless and restful as well, but like you have moved away from them to a simpler setup. Just using a simple flask app + blueprints with a few extra error handling / cors bits and a custom response class for json. I'm very happy with marshmallow too.