Creating video from images with different SARs with FFMPEG
You have to set all the SARs beforehand. So, [1:v][sar0]blend=.. will fail because 1:v still presents a different SAR. [sar1][sar0]blend=.." will work and so on.
It may just be easier to do this in a separate command preemptively for each input image and use the output:
ffmpeg -i watermark-5.png -vf setsar=1 watermark-5n.png