I've been using Unity in a similar way but with Luau (a gradually typed Lua by Roblox) for the API. It's been very educational to treat Unity as a box like this, and I've been making the Luau API very high-level, often following Roblox patterns (more coarsely grained).
As a side-effect of this separation, it's also refreshing to have the option to make everything work with other engines.
Making Unreal rendering layer would obviously be a huge task. Our current solution is overfit to Unity since the separation was done for ease of development and testing, not for support of different renderers. However, the communication between core sim and rendering is mostly based on (custom) events.
Sim runs in a separate thread. Once it is done, renderer can read data (sync) and launch the sim for another iteration. We do this at 10 Hz. In the meantime, the renderer should interpolate all moving objects from state (t-1) to (t) so that movements are smooth (say for target of 60 FPS).
For example, a `TreeRenderer` can listen to `TreeManager.TreeAdded(TreeId id)` and `TreeManager.TreeRemoved(TreeId id)`, and handle rendering of the trees however it pleases. Trees don't move so this one is easy. `VehiclesRenderer` has to also update a vehicle state on every sync.
One can also inspect sim state without usage of events. For example, our debug renderer just draws a picture of a game state by simply looping over all entities.
Reality is of course much more complex. If you happen to own the game, our DLLs are non-obfuscated and quite easy to read with tools like ILSpy :)