Got to 10,000 and figured I'd go on for a while since I was listening to a podcast.
I stopped after 20,000 because I got buyers remorese. Maybe I'd feel differently if I got to later sections, but the most efficient method for earning money for me was to just buy load upgrades and lots of basic food. I even wonder if upgrading stamina is that worth it ebyond a point, since you still need to refill it regardless.
The thing that takes longest is trudging back to the start, so the second pickaxe upgrade isn't much of a boon. Actually, I found the level 2 pickaxe to be the most fun an easy to use. Something about the power-pickaxe always felt like it was quicker and harder to land. Could just be psycological, but that's how it felt.
Maybe if it upgraded the quality/ammount of ore you mined instead. Then that would also incentivise load upgrades since you wouldn't want to squander a good load if it doesn't divide so evenly into the haul.
Also some movement speed upgrade would be sweet.
And maybe a compass. I managed to keep myslef oriented by having a main tunnel with offshoots, but one time I got spun around and panicked a bit. I dunno if it's better to keep that aspect of self-orientation since it made it more interesting, or let the player buy a compass so they don't get lost as easily.
All in all this is a great game to relax and chill out to. I think it just needs some extra wrinkles and tweaks. I would have played longer if that Pickaxe felt like it was worth the 10,000 sink. The cost needs to really suit the item so it doesn't feel like a mistake/set-back.
(Oh, and this might be a me think, but I'm not a fan of queing to this extent. At least not with this movement speed. I'm perfer a hold down button to keep moving if anything. It'd save my index finger RSI.)
Playthrough: