Reading through the thread again I’m wondering if you are not expecting the impossible.
Your incoming image is 16:9. Your outgoing image is to be 21:9. They are different shapes. So as László commented either you show your full 16:9 image uncropped with black bars at the sides or you crop the 16:9 image at the top and bottom so it is the same shape as 21:9 and fills the area.
Alternatively stretch out the 16:9 image to 21:9 so the entire image is included, it fills the screen, but it is distorted.