What is the fastest way to build a pawn square?

This is a silly question that requires some explanation.

What I mean by "pawn square" is a 3x3 square using all eight pawns with a space in the center. As a cherry on top, a knight sits in the center square. Like so:

(The pawns don't need to sit on any specific colored square. The corner pawns can sit on light or dark squares.)

In this game, Black will move wherever White asks them to move to facilitate construction of the pawn square.

What is the fewest number of turns it would it take for White to create this formation from the opening?

I managed to do it in 20.5 moves:

Wow, nice! A stylish way to do it, too. I like how the rook buzzes the white pieces at the end.

I got one in 19 moves.

I wonder how much more difficult it would be with a rook or a bishop in the center instead of the knight.

Nice! I thought I had the fastest, but this saves 2 white moves.

I would think R or B on d5 would be the same length; both pieces need only 2 moves to reach d5, as the N does. Just have to time the pawn moves carefully to let them through.

Rook on d5:

Bishop on d5: