It would be nice if the cobblegens could be harvested faster with a pickaxe, as it's pretty slow at the moment.

This is correct, it's a design choice, they are meant to be slow, but can be picked up by anything. And also its easier for a user to add them to pickaxes then to remove them. You're more then welcome to use a datapack and add the tags yourself.
data/minecraft/tags/blocks/mineable/pickaxe.json

{
  "values": [
    "cobblefordays:tier_1",
    "cobblefordays:tier_2",
    "cobblefordays:tier_3",
    "cobblefordays:tier_4",
    "cobblefordays:tier_5"
  ]
}