First of.
Your example is even worse than non functional - you are affecting the whole material at once. Regadless of what or where the saturation should be increased. Which is not at all likely to be everywhere.
Secondly:
You can’t just saturate that way - its nonsensical.
You want to bias the saturation based on a color channel or a hue to achieve any sort of decent effect.
For example: fragment shader - Hue, saturation, brightness, contrast effect in hlsl - Game Development Stack Exchange
Thirdly.
Try it. You won’t be able to modify other layer by a layer if you use the landscape’s default stuff unless you: a) know what you are doing. Which you prove time and time again you do not.
b) use specific layer types, since blend will not work to override a layer. c) actually act on the correct output, and d) use a custom layer node.
Try it. See just how “trivial” it is.
Particuarly vs copy pasting code into a custom node.