Hey @AndyPicamaze - I'm looking for an API-based solution here actually. The big tedium comes from generating field data for each individual product, customer, etc. that I'd want to test with, which makes CSV imports still a process of generating test data before I can actually conduct the import.
There isn't a direct way in the API to 'autofill' random data. You could check out the Data generator in the Shopify Developer tools for macOS: https://www.shopify.ca/partners/blog/developer-tools, but this isn't a maintained tool any more. You could also look into the Faker library (ruby / python version) for generating sample data.