I did that for my Unity games like a decade ago, you can too :) Requires a design that has one core and then using Unity basically as a renderer/viewer though, but is a relatively easy design actually. And yes, automation and DX gets so much easier. I think Captain of Industry is built like this too, remember reading about it here on HN from the developers in some comments.

Same here, in Godot! This is a common practice for games with performance/networking constraints too. For example building a deterministic sim in a regular engine tends to fail without custom primitives or a nice library to handle some of the intricacies. By moving non-display code out of the engines you can reliably build and control all of this.