...and now the results are very poor...
And in which way exactly? How do you judge on quality?
My ... settings are ... 792x582 (same size as source)... My Preview File Format is the default I-frame only MPEG.
Your frame size is not good for previewing in PrPro. MPEG 2 uses macroblocks of 8x8 pixels for compressing data. And now what you get with your frame size: 792 pix / 8 = 99; 582 pix / 8 = 72.75
My export settings are...
See also this The Video Road blogpost on Understanding Colour Processing and Render at Maximum Depth option benefits.