You could try and open an issue on GitHub for gstthetauvc. The developer definitely has a lot of expertise.
As you can stream to vlc, the problem is likely with some other software in your workflow.
For example, does your OpenCV have the gstreamer backend? Check it with the following:
You can also try searching on the NVIDIA formware for “ERROR: pipeline doesn’t want to preroll”
gst-launch-1.0 thetauvcsrc mode=4K ! queue ! h264parse ! nvv4l2decoder ! queue ! nv3dsink sync=false working well.
If that thetauvcsrc is working, can you explicitly specify nvv4l2decoder and nv3dsink in this pipeline?
VideoCapture cap("thetauvcsrc ! decodebin ! autovideoconvert ! video/x-raw,format=BGRx ! queue ! videoconvert ! video/x-raw,format=BGR ! queue ! appsink");
Does the pipeline above not work?