[UK-AL]

"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.

[Quarrelsome]

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.