Not trying to make you change anything, but did you consider having natural cave entrances? That way players can access your mod at anytime, but have to work their way up the tech tree to be able to make entrances where they want them.
I've been thinking about something like that for a while. My initial idea for the cave entrances was a "mineshaft" building that the player needed to be nearby to enter the cave, but it got really annoying having to run back-and-forth just to go up/down. I might bring something like that back at some point that starts the player their and then the tunnelling-equipment would be a QoL item, but for now I just want the drill to make it convenient.
At this point in the mod I want the caves themselves to be the challenge, not the movement between them and the surface. Good suggestion though! If you have more feel free to join the discord, I'd love to hear them (or any other issues)
https://discord.gg/UsAw48hP