We're planning on extending the game with more islands/content/length. You can think of this game as a work in progress (or endless early access title).
For pick axes, 1 stamina per hit is actually very good. The stone pick axe takes 3 stamina per hit if you hover over it, so by not using the steel pickaxe, you're hurting yourself.
I'm glad you enjoyed the game though! There will definitely be many improvements/new islands/features/content and even secret dungeons buried underground like you were hoping for!