Thanks so much for playing, and for the kind words!
The Diamond Pickaxe is unlocked from treasure chests in the Granite and Magma zones, though the drop rate is very low (0.1%)
Appreciate the bug report about the Crystal Circlet visual effect. That's definitely a bug. I'll try to come up with a fix in the next update.
Achievements are something I definitely want to expand, but I'm trying to find the right balance. I want them to feel earned and meaningful, like Heavy Drip. I've been thinking about a 100% completion achievement too, but I have not decided how I would like to implement it. For example if someone unlocks everything now and gets that achievement, should they lose it when new content is added? Or should it remain a permanent badge? Still deciding which is better...
As for Nix, it's a bit of an obscure unlock by design. She's sort of like a challenge turtle, where unlocking and reaching water with her is more difficult than the rest. I have included a small hint in the starting screen's game tips :)
Thanks again for the thoughtful feedback. It really means a lot!