Hi,
Please check if you can apply your use-case to this sample:
Displaying to the screen with OpenCV and GStreamer - #9 by DaneLLL
And use nvoverlaysink in cv2.VideoWriter() like:
gst_out = "appsrc ! video/x-raw, format=BGR ! queue ! videoconvert ! video/x-raw,format=BGRx ! nvvidconv ! video/x-raw(memory:NVMM),format=NV12 ! nvoverlaysink "