Question How to cull objects behind ui

Bottom quarter of my screen is covered with UI all the time how can I stop things behind it from rendering?

My ideas:
1)Customize culling: But how could I do that? I mean can I influence built-in culling? There is a Camera.cullingMatrix could this work? Or do I need to roll my own culling algorithm?

2)Can I change camera screen size so it skips bottom quarter?

3)Maybe I could do a scissor test with a command buffer that would save me the execution of fragment shaders.

4)Maybe I could render camera to a smaller render texture(without bottom part) and then output that render texture to the camera?

What direction should I take to solve this problem? Drawing a quarter of the screen without needing is really wasteful.

 

Number 2: https://docs.unity3d.com/ScriptReference/Camera-rect.html

 

But will this actually reduce resolution for gpu calculations or just scale them down to fit the smaller rect. Because when I change rect size I get the same frame result just scaled down. Maybe I also need to change my other camera settings with the rect.