Thanks so much for the thoughtful feedback! Cows have a 50% chance to spawn each time you return to the surface, so popping back up more often can help speed things along if you're running low on leather.
As for Quartz, they don't naturally spawn in the Magma Zone. For Nix's unlock, the intended path is to bring quartz down with you into the magma rather than hunt for it there.
Serenitrove is still under development, and more features will be added from time to time, thanks again for your feedback!