Title says basically everything. I've tried changing the block's hardness, but I haven't found any way to do so, yet. Using Forge 1.14.4-28.1.87, if that matters.
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EDIT:
I found a solution.
Basically, you replace the registry entry for bedrock (or other block you want) with your own, change the properties, and then also implement a PlayerEvent.BreakSpeed handler to check if the tool is valid. At least, that's how I did it and it works. If I'm being an idiot, please say so.