[sbov]

Procedural/functional code. Splitting your models up more by features than high level things (e.g. having a separate CustomerBilling rather than putting it in Customer). Component based architectures. Service layer to coordinate models. Etc. Also, this is/was a common problem (at least, the god class aspect) in games that the industry has slowly solved over the past 20 years, so that's another place you can look for inspiration.

Further, unless you know you're going to scale in the beginning, I would recommend refactoring/evolving over time. It doesn't do anyone any favors by having 10 models to represent your Customer if your Customer is already relatively thin.