Very fun game! I love the sound design, graphics, concept. Here are my notes in order of what I personally found most important:
1. I would love some visual indicator that the treads are working when I'm moving forwards or backwards. Especially when I'm wiggling back and forth trying to get backwards through a tunnel I really wanted to know when my wheels were actually finding purchase.
2. The drill's tendency to (as far as I can tell) jerk towards the path of least resistance can be very frustrating, especially in cases where I am pointed directly towards a motherload of gems. When missing a gem by a little bit can mean a full minute of careful adjustment or doubling back to get it, missing one because of the drill's deviation hurts.
3. Since the drill upgrade system can often force me to double back, I wish moving upwards was easier. It would be more satisfying if the drill bit itself could pull you forwards, at least when moving up. I had several frustrating moments trying to find purchase on enough soil to move back up, but since my drill bit was the only thing that could reach soil, I could only watch in dismay as the bit ate up the path I wanted to take upward, and I dropped back down to where I started.
4. There were very few moments when I felt the explosion was helpful. Perhaps if it at least exploded around the entire drill, or perhaps only the back section. That at least would have saved me from a softlock I got at one point, where the back of my drill got stuck in a small gap while the rest of my drill hung down in limbo.