At first sight it seems like you still have to write a "schema" for the SQLModel based on your SQLALchemy model - so basically, two sources of truth.
If you edit your SQLAlchemy model, you'll also have to edit your SQLModel.
sqlalchemy-marshmallow allows you to build your schema based on your SQLAlchemy model.
Other than that, I'm still somewhat intrigued.
Thanks for the suggestion.