FFmpeg issue with aspect ratio
Use

"[0:v]scale=iw*sar:ih,setsar=1,pad='max(iw\,2*trunc(ih*16/9/2))':'max(ih\,2*trunc(ow*9/16/2))':(ow-iw)/2:(oh-ih)/2[v0];[1:v][v0]scale2ref[v1][v0];[v0][v1]overlay=x=(W-w)/2:y=(H-h)/2[v]"

First, the scale and setsar make sure the image becomes a square-pixel input. Then the pad checks each dimension and pads to a 16:9 frame size.