I don't know how long you have explored for veins in total, but 20 pickaxes is not a lot.
That was just at y50 for apatite... I've found virtually all deeper ore types at y20. Got real lucky and first thing I found was diamond.
I don't mind mining at the low y levels because there is a lot to find there, but not many ores spawn between 40-60 so it's mostly just hacking away at stone.