Can you provide a log? that cpu is a beast and should be able to handle 1080p60 encoding but this ofcourse depends on the cpu usage preset you used and the game you are trying to run with it concurrently.
Why on earth should anyone use 1600x900 as a base resolution and than upscale it to 1920x1080?
In my opinion you should use your monitor resolution as base canvas and set the output resolution to the same or lower resolution, but not higher.
If your CPU struggles to encode 1080p 60fps, simply reduce the CPU load by switching from x264 medium preset to faster or very fast preset.
When you use the Streamlabs Branch of OBS, you should ask Streamlabs for support.