The way I do reflections for flat & clear surfaces (such as mirrors or a settled lake) is to use a world-space cubemap and reflect only characters or other objects in real time (often using simplified materials and meshes for the reflected versions of those meshes.
GTA V does this for their city, they make a really low quality/resolution version of their city and use a render-texture type system then move the render-texture target in tandem with the players camera, it can give extremely realistic results. There’s no reason as to why you couldn’t include other dynamic objects in this.
Bare in mind though that you are taking on the full cost of rendering these scenes so keep material/shader complexity waaay down and shadows off/disabled for the render target.