Now all scale, pad and aspect is done inside the filter chain which avoids the conflicts you were experiencing.
1. scale the 1920x1080 source to 768x432 dimensions (square pixel 16:9)
2. Pad this to the output size of 768x576
3. Set the aspect ratio to 4:3
NOTE: When using libx264 in recent versions of FFmpeg, the -vpre option has been replaced by -preset. Modify the code or update your FFmpeg as required. Also, I left out your audio options for simplicity.