Your thinking is sound. At the end of the day Rails default fixtures is nothing more than some code that reads yaml files and creates records once at the start of test suite run.

So you can definitely use FactoryBot to create them. However, the reason I think that's rarely done is that you're pretty likely to start recreating a lot of the features of Rails fixtures yourself. And perhaps all you need to do is to dynamically generate the yaml files. Rails yaml fixtures are actually ERB files and you can treat is an ERB template and generate its code dynamically: https://guides.rubyonrails.org/testing.html#embedding-code-i...

If that is flexible enough for you, it's a better path since you'll get all the usual fixture helpers and association resolving logic for free.