Thanks for the feedback. Glad you like the overall idea. Yeah, the pacing and grindy aspects are something to think about.
Regarding drilling upwards, that's an intertesting idea I considered during the development. My initial concept didn't even allow for drilling while in the air. My original concept was that there's always downward progression. You can strategize on how much down you go and how much left/right you go, but overall you're always going down. Going up would change that directionality. I will have to implement that and playtest on how it affects the gameplay.