this is how the starting position would look like 
The next is an example of a game after 7 moves in which the player with white pieces has the idea of getting a mass of pieces and pawns in which to shelter his king, while the player of black pieces explores possibilities that allow him to get better pieces quickly
1.e4=F e11=F
2.d4=F g11=F
3.c4=F Fgf10
4.f4=F n11=F
5.g4=F F9=I
6.g3 n12
7.Kg2 Fm10
The idea of this chess variant is that both players start with a very weak setup (only pawns) and these become better and better pieces as they move forward on the board. To make this promotion race more interesting, we will use an “xxl chess” board. The players' kings are placed on h1 and h14 just like in a normal Xxl chess game. In fact, in the second Rank (13th for black) it's all pawns, just like in normal Xxl chess. The big difference starts at the first rank(14th for black pieces) since all pieces (except kings) are replaced with more pawns. So now each side has an army of 27 pawns, 13 on their first plus 14 on their second.
Pawns will be able to move two squares forward on their first move.
When any of your pawns reach 4th rank (11th for black) they will promote to a Ferz.
When any of your Ferz reach 6th rank (9th for black) they will promote to an Alfil (elephant).
When any of your Alfils (elephants) reach 8th rank (7th for black) they will promote to a Knight.
When any of your Knights reach 10th rank (5th for black) they will promote to a General.
When any of your Generals reach 12th rank (3rd for black) they will promote to an Archbishop.
When any of your Archbishops reach 14th rank (1st for black) they will promote to a Queen.
Pawns first upgrade to a Ferz to gain a bit of mobility so you can maneuver with them better. They then promote into a bishop, which with its double jump can easily get to the next promotion square to improve itself to a Knight, a much better piece which also can reach the next promotion square in just one move. As players start getting Knights in the match, they will now be able to skip promotion squares. Although it is not a very attractive idea, since the following promotion only adds mobility to our Knight, by turning it into a General. Same thing when promoting to an Archbishop. Pieces are just getting better and better. 12th rank (3rd for black) promotes to an Archbishop, but I think a Chancellor could be another good option to analyze.
I think that final promotion must be to a Queen and not an Amazon, because that could give players a reason to keep some knights, Generals or Archbishops on the board so they can combine their peculiar horse jump with the powerful Queen.
In the image, I remove both pair of white and black pawns that should be in a and n files, and use those files to show the squares and how each player would promote their pawns and pieces.
Since it is not possible for me to modify the rules in the custom mode as needed, I cannot test my variant like you request but I hope you will still like the idea.
Thanks for reading.