Specifically I want to add the block byg:soapstone from the mod Oh The Biomes You'll Go v1.4 Fabric.

I asked in the modpack "Another Quality Modpack 2" Discord server how to do this, and I have been told to look into Tags but I don't know how to do this.

Is that something I can do with datapacks, or is this a thing you need to change or is this a problem I need to address with the Devs of Oh The Biomes You'll Go?

The blocks it works on are hard coded for now, although I look into the possibility of moving them to a tag.

hold on, what do you mean Mining Speed Multiplier? Hammers just break blocks, there is speed increase.

Not going to pursue.