Super fun variant! I think doubling the rooks and never placing knights is the optimal strategy. Maybe knights should be cheaper. Also interesting being able to guard the king with pawns on rank 1. Bot isn't very challenging once you figure out the mechanics, I'll have to give multiplayer a try. It would be great to have online multiplayer, since I doubt local bullet works very well.
The most unbalanced card seems to be the one that gives you the strongest piece in possession for free. Very broken if queens are still on the board. I used this card to get an extra king, but it selected the pawn instead (I guess king isnt a "piece"?)
Promotion screen looks weird, but it works ¯\_(ツ)_/¯