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):
1-Pickaxe
2-Axe
3-Shovel
4-Sword
5-Shears
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).
I will consider improving this in the future updates.