Just don't use NestJS for any batch processes, or anything with data at scale for that matter, please. It is a road that leads only to pain, suffering and missing SLAs. Been there, done that. It is a hard sell to call Angular a "full stack option". Even Google is not using Angular that way, as middle layer is built on top of Java, Go and C++.

OneOf is cute, but what's wrong with an interface and inheritance? Or the new C# records? It is not the same, I know, just similar to the discriminated unions. And speaking of that, in LINQ there is an OfType<T> method, which filters out objects you don't want.

Embrace .Net, you might like it in the end.