You use a static typed language for guardrails but then you throw out the guardrails of a database schema? Seems like those two decisions are directly at odds. Without a db schema, you still have to worry about migrating data at runtime or otherwise. Removing the schema just shifts the pain doesn’t remove it, in my experience.
The schema in Modelence is defined in your code, and at the moment the only use case where it's not enough is if someone directly modifies the database data externally. We're not against having the native MongoDB schema as an extra enforcement, the only reason we haven't added it yet is because it requires extra work to carefully sync both. I believe at some point we'll add it as an extra layer to prevent corrupting the data by direct modifications.