I believe you can build great abstractions in this kind of software, but if you want them to survive you gotta keep them any of that away from anything involving the business logic itself. You can only do this on product-like things: authn/authz, audit logs, abstractions over the database (CQRS, event sourcing), content/translation management, messaging infrastructure, real infrastructure. As soon as you allow anything from the business itself to affect or dictate those abstractions, you get shit again.

You're right that the business logic is gonna be messy, and that's because nobody really cares, and they can offload the responsibility to developers, or anyone punching it in.

On the other hand, separating "good code" and "bad code" can have horrible outcomes too.

One "solution" I saw in a fintech I worked at, was putting the logic in the hands of business people itself, in the form of a decision engine.

Basically it forced the business itself to maintain its own ball of mud. It was impossible to test, impossible to understand and even impossible simulate. Eventually software operators were hired, basically junior-level developers using a graphical interface for writing the code.

It was rewritten a couple times, always with the same outcome of everything getting messy after two or three years.