You identify what customers want to customize, and provide a way customizing that. Some customers want 10 courses per student, others 5 courses. You could easily model that in a domain model.

If you want something radically different, then are you even developing the same application anymore?

By the way i've never heard anyone call the core entities of an application "Currency" before, where did you even learn that? I imagine that could be confusing.

I also imagine having an application where you have build custom methods for each customer who wants something slightly different is terrible for codebase maintainability.

They're currency because when you buy something from a shop your money doesn't start telling you what you can and can't do and it travels all the way through many layers of worlds. It is the core and its a good word, use it.

I make a career out coming to companies that should have (according to you) made radically different systems but didn't. Most companies don't find it economically justifiable to re-write based on slightly different customer requirements and most companies have many deployments, not just "one perfect one" that most modellers appear to imagine. If your design doesn't have the flexibility of change designed into it you're going to struggle when you get a second client.

> I also imagine having an application where you have build custom methods for each customer who wants something slightly different is terrible for codebase maintainability.

Yeah of course its a massive pain in the ass but it enables you to sell more.

"Most companies don't find it economically justifiable to re-write based on slightly different customer requirements and most companies have many deployments,"

Then don't. Make the domain model customisable with various options.

Having specific methods for specific companies seems a lot worse.

I build SAAS applications which have various customization options customers can do. I don't really have a problem with it. I've designed a clean way to customise.

I do more enterprisey stuff so our customers are a little less accepting of model restriction. I find a service based architecture more flexible as its pretty easy swap out services that just work off a common currency that define different or customisable behaviour. The sort of situation where an entity is performing business logic makes that harder do as opposed to swapping out the service.

I made the same mistakes as OP and learnt the hard way. I encourage you to explore a greater purity in your models in the future because I genuinely believe it leads to better code.