Turning off the OSD Output setting on the camera should be all you need to do for a clean HDMI or SDI output.
Just to double check is the issue that you are still seeing STBY and REC on the built in EOS C200 screen, or is it showing on your connected SDI monitor? If it is still showing on the built in EOS C200 monitor that is okay. The OSD Output setting only applies to the outputs HDMI and SDI outputs to an external monitor. That setting does not turn off the onscreen displays on the built in monitor.
Thank you! Unfortunately that is a bug with the C200 and SDI output, it wasn't made for a clean feed. I spent half an hour yesterday on the phone with a technician at Canon who was experimenting with a monitor on his end and he saw that as well. I'm hoping they can provide a fix. Thank you much for your time, Hazel!