Educational project - Need to Enhance Unity Scene Realism and cut overall size of the project

I have a compact Unity 3D scene that shows a real school building and that already works functionally but still looks a bit flat and the overall size of the project in proportion to the scene is way too high. My main goal is to shrink the overall project in order to make something usable in webgl by mobile online. Also an enhance of the visual quality and the surrounding so that the looking out of the building window and on the external premises of the building it would look good showing to the player the feelings to be in the location.

I’m targeting mobile so the size is important.

Keep in mind this is an educational project so budget is not high and super quality is not important.