I'm primarily an Unreal Engine developer, but both engines share a lot of similar concepts under different names. In Unity HDRP, I'd suggest experimenting with Exponential Fog and volumetric fog settings. I think it could add a lot of atmosphere and depth to the scene.
That said, it already looks great as it is. You should be proud of what you've achieved so far, you've done an excellent job
🧡🧡🐘