Just wanted to say, this is an amazing piece of art and design. It's just so... elegant.
My one suggestion is maybe a mulligan for the initial draw, or a way to manipulate your initial hand. The reason being, many builds snowball, so a terrible initial hand can lose you the match. Outside of the tournament, and challenge run attempts, I found myself resetting if it didn't get a good starting hand, as this felt faster than playing a middling hand. I sometimes overdid it, and tried to force a semi-rare initial hand. (optimizing the fun out of the game).
P.S. Does the efficient achievement require being in challenge mode vs casual mode?