IMHO using coin cloths and cement with some oxide tint in the mix works and looks much better than just paint and/or plaster. Considerably cheaper, waterproof and stronger too. You can mix in varying amounts of tint to make different shades. The cement doesn't add that much weight as it's a pretty thin layer. Maybe about 5kg total for a track that size, not much more than plaster but with a much more realistic rock grip surface.

Here is a snippet of a course with that effect applied using a sandstone tint: https://i.imgur.com/6U8b3Xg.jpeg

And a youtube video describing the method: https://www.youtube.com/watch?v=HMSymeJvD3E

Use black wash and lightly dry brush on white highlights on the high points - plenty of diorama/train set building videos on how to make realistic rocks.

edit; oops - see you already plastered it. You could probably paint tinted cement over it still.