Best hardware for 4k video SDI input / run OpenCV for overlay / 4k video SDI output with reduced delay?

Good afternoon,

I am trying to catch a 4k live broadcast video thanks to an SDI connector, add an overlay thanks to OpenCV / Python script, then re-send live by SDI output,with very minimum lad/delay.

Unfortunately the new OpenCV Oak has no camera input

Would you have any hint please ?

Thank you