I think you should approach this a different way.
Using a displacement map to generate your whole world probably isn't a good idea. It will actually lead to lower quality and worse performance.
When making a game, the usual process goes something like this:
- Make individual models (assets) for the game. This would mean things like houses, trees, walls, etc. would be individual objects. I would imagine that is how you made your scene initially.
- Add materials to these objects. When you export a model to a game engine from Blender, the only information about how the objects look is stored in an image texture. Thus, anything that can't be stored in an image texture (fog, glowing objects, anything that has to interact with the world in real-time like a reflection, etc.) usually shouldn't be done inside Blender, at least not in the way that you'd normally set these things up in Cycles.
- Export the individual assets with their textures to a game engine, and build/rebuild the world in the game engine.
When making the individual assets for a game, there is a general process that is usually followed for this too. I'll use a house as an example since you have one in your scene.
- Start by making the 3D model. If your goal is to make the house as realistic as possible in the game, you would make a very detailed house model in Blender. If you're not going for really high detail, you can make the house with less modeled detail in Blender. This can still give really good results too.
- Texture the house, using whatever methods you want. You can use image textures, procedural textures, whatever it takes.
- If you made the house with more modeled detail than you think the game engine can handle, at this point you would "retopologize" the house, meaning making a lower detail version based on the high detail model.
- Bake the various kinds of detail into image textures. I think this is the step that will help you the most. Cycles has a bunch of options to bake detail, including normals, roughness, color, shadows, and many more. These settings are all found at the bottom of the Render Properties tab (the Camera icon) on the right side of the screen. If you have a high detail and low detail model, you can bake the normals from the high detail model to the low detail model, making it look like it has more detail than it actually does.
- Once you have an individual object made, with as little modeled detail as possible, and you have all the textures that you need for that object, you would import it into the game engine. This process can vary depending on the engine, so if you need more information on how to do that I would look up tutorials or other information on the specific game engine.
I mentioned that using a displacement map will give you a lower quality result, so I'll explain that a bit more.
Let's look at a simple cube. A modeled cube has only 6 faces, which will be converted to 12 triangles when rendered. No matter how close you get, the faces will appear perfectly flat, and the edges nice and clean. If you use a displacement map, you will need a very dense mesh to displace to get even close to the same amount of definition. If you wanted to a get an edge that appears perfectly straight and sharp, you would actually need a mesh that is so dense that each face ends up smaller than a single pixel. Otherwise you would get jagged edges, rounded edges, weird faces on the edge, or other issues. For certain things displacement maps can be great, but in general, a properly modeled and textured object can be more detailed with less faces.
If your concern is realism while keeping it easy to render, I would look up more information on modelling game assets, baking details such as normals, and how to setup game worlds using whatever engine you choose.
I know there's a lot of information here, and this still isn't a perfect guide to making games, but it should give you enough to get you started and help you find more information.
Hope this helps!