I would define the complete structure of the project.
- Is it sfdx project?
- There will be a lot of classes - should we have them with the namespace as the fflib does it?
- I would also consider an option to have all classes divided but implement a bundler to compile them into one static Assert class. In that case, the library development won't be affected as everyone works on different classes (less merge conflicts) but eventually the lib dist is only one class - Assert
I would define the complete structure of the project.