Is there a way to upload a schema, using json or something so that I don’t have to manually use the UI for large schemas over and over again?
You have an API available, check it out here-
Mockaroo - Random Data Generator and API Mocking Tool | JSON / CSV / SQL / Excel
You have an API available to generate data with options to reference an existing schema or send your schema in the request, check it out here:
Mockaroo - Random Data Generator and API Mocking Tool | JSON / CSV / SQL / Excel