Your version 1 is similar to my top level comment. Note that 4 bits is enough to encode color, type, and include special types for en passantable pawn and castl

Your version 1 is similar to my top level comment. Note that 4 bits is enough to encode color, type, and include special types for en passantable pawn and castleable rook, so you keep a clean 24 bytes. (Compressing the two special types into one special option, decodable based on position, can get just under 23 bytes.) For version 2, note that 12 promotions can happen, since one capture (white's B pawn taking black's A pawn) can open up the promotion path for 3 pawns (white's B pawn, white's A pawn, and black's B pawn). Two bits per pawn and 5 bits per other piece start at 14 bytes, but can go as high as 17.5 bytes. (Promoting 12 pawns with 0 other captures would be pretty ridiculous as a game, but technically possible...)