Hi Bernd,
Your post caused quite a stir here!
One thing that might help is this tool that I just wrote:
It requires ROS 2 Galactic because it uses the new generic_subscriber to receive messages without deserialising them. This will, I think, allow it to measure bandwidth topic with minimal overhead. (If it can be improved, please do send in suggestions!) It should more efficient bandwidth measurement than the Python-based ros2 topic bw tool does, which could be useful. Maybe.