At the moment a custom block can only be harvested with a pickaxe, an axe or a shovel.
However, in vanilla minecraft, some blocks can only be harvested with a sword or some shears. Some blocks can even be harvested with anything (even hands).
So my idea is that instead of choosing a single tool that can harvest a block, you can choose a list.
Here are the tools you can choose in the list (you ca choose more than one of them):
If the user chooses no tool, then it can only be harvested with special tools (and, if there are no special tools that can harvest that block, it can't be harvested).