Agree with others on the indicator as to when coal was mined/how much the player currently has! Another point for me would be an accessibility option to change mining from each click, to being able to hold the mouse down to mine! (My fingers are awful these days and so couldn't play for long).
The idea and look of the game are amazing, great job to you both! Especially loved the touch of the character's head being the Godot loading icon :)