I got absolutely addicted to this! It was a lot of fun.
Like commenter TripleRainbowBaka suggested, I think it would be super cool to have different worlds to go to.
Following up commenter spiritbx, it would help a lot if the rocks had numbered health bars. When I first started playing, I thought the +10s popping up were how much resources I was getting. A health bar and changing the + to a - in the popup would help clear up any of that confusion. I agree with what they said about the space bar too. My hand started to hurt from pressing so much. It would be nice if I could hold down the space bar to mine. Maybe you could add a mining speed upgrade that determines how fast the rock is mined while holding space bar.
I never found myself upgrading the shift length. I never saw the point considering I stayed in my previous position and kept all the resources. Maybe you could require the player to sell their resources before the shift ends to make their money? I feel like that would be a bit tedious though. Another reason I never upgraded the shift length was because it would mean I upgraded less frequently. You could add a bed so the player can end their shift early if they have the shift upgraded a ton.
I also never upgraded the speed stat. There was never a time crunch to get between the rocks and the sell stand. This could possibly be resolved by adding my suggestion in my shift length paragraph about losing the resources if you don't sell them. I think it would also be cool if the rocks despawned after being mined and they would randomly spawn in an area, incentivizing movement around the map. Out of all 3 rocks, I think I only ever mined the top one lol.
This game is super cool and incredible for your first game release. I am excited to see where this project goes!