Time for a new webcam. This time it needed to be FullHD but also be able to send the full stream via USB without killing the quality or performance.
So my choice is the Logitech C925e which supports h.264 video encoding 1080p/30fps.
Excellent low light behavior stable colors. No focus pumping, everything crystal clear. Good enough for work. That 4K stuff is simply too expensive and not yet required.