Yes, building an extension is something we are looking at, and I am glad to hear that you would be interested in this kind of mixed use case. Would definitely make things interesting, and more performant.
What about optionally validating some columns with jsonschema? Perhaps using https://github.com/supabase/pg_jsonschema - is using other postgres extensions supported in FerretDB? (if not, maybe it's feasible to incorporate the code of pg_jsonschema in FerretDB?)
