Schema-less is a bit wrong term, since you always end up with one but, indeed, schema-on-read vs schema-on-write discussion for individual use cases is far from a settled thing, even though the zeitgeist is that schema-on-write won sometime in the 70s.

You say "prototyping", but I would generalize that to "faster evolving" in the long term. Of course, not without tradeoffs.

I personally have zero issues writing a custom marshaller when needed for any the schema-less document collections I have. Constraints your application has on the data are a superset of db schema anyways.