We want to be able to level-up our testing and have found ApexMocks as a very helpful library that we can use for this purpose. I'm not sure if I can cover ALL the use cases but I hope it can help us test page controllers, triggers, relationships, formula fields, utilities, callouts, etc. The problem we are dealing with is that the library has 30+ classes and looks like we will have to add it to our managed package to be able to use it, which increases (or bloats) the code base, which is not ideal.
Now the question is: Can we use this library without including it in our managed package? Or is there a better alternative?