How to order files by "full number name"?

You could do a few different things:

Rename your files with padded zeroes:

0000000.png
0010000.png
0100000.png
1000000.png
1005000.png
1010000.png

Use the version sorting option:

file_args=()
while IFS= read -r file; do
    file_args+=( -i "$file" )
done < <(ls -v *.png)
# ..........^^
# or
#      <(ls *.png | sort -V)

then

ffmpeg -framerate 2 "${file_args[@]}" test.mp4

1-liner:

cat $(find . -maxdepth 1 -name '*.png' -print | sort -V) | ffmpeg -framerate 2 -i - -vf format=yuv420p -movflags +faststart output.mp4

Many players won't like 2 fps, so if it's not working add -r 10 output option and it will duplicate frames to compensate (output will look the same, duration will be the same).