I think you're underestimating what can be done with the industry standard engines. From a rendering perspective modern game engines can handle so much stuff th

I think you're underestimating what can be done with the industry standard engines. From a rendering perspective modern game engines can handle so much stuff that it will fill your screen entirely. Just look at UE5's Nanite which supports ridiculously massive scenes and splits the frame into roughly pixel-sized triangles to render at interactive frame rates. If you do end up in a situations where you have more objects than is suitable for even these systems to simulate you don't need to throw the baby out with the bath water. Create your own managers running whatever simulation you wish without tying the units directly to the most obvious game engine entities, with whatever scheduling you need to make the updates work with the compute budget you have. Use the engine entities to represent whatever higher level object makes the code manageable and easy to work with. Then you can surely leverage the game engine for input handling, rendering, and a lot of your logic, just writing your own code for whatever challenges are unique to your game and where the standard approach for that engine is unsuitable.