I've unlocked my cart's max capacity of 30 ore and am only at the Spooky Spires, and already the mix of copper, iron, and silver is limiting how many bars of anything I can smelt, if I don't play the game of “manually remove the ore, one by one, and dig some more” to get everything grouped into fives. And if each higher level brings with it an increasing variety of ore, this is going to become even more frustrating. Especially so, for those whose carts are not maxed out!
So, I'd like to suggest that the cart's max capacity be increased to 50, to help accommodate the mix of ore a little. This suggestion would pair well with the one for lowering the cost of expansions or offering pebble prices (at least for mining, anyway), as the new expansions can either entirely be priced in increasing amounts of pebbles, or mostly priced in pebbles with just a couple being cheaper moonstone prices.