I’m do not think you always want to hit them hard. If our understanding of physics is right, it doesn’t matter whether you hit them or they hit you.

the two conkers hitting each other harder likely will lead to an earlier result, but it will also favor the conker that can withstand few hard blows over one that can withstand many softer ones.

So, assuming you can somehow judge how well your and your opponent’s conker do in this regard, you may want to go either for brute impact or for many rounds.

> What's the optimum strike angle to crack a chestnut along its natural cleavages?

I think that’s more important. Even idealized conkers are fairly asymmetrical, so possibly, the ‘bottom’ of one hitting the side of another is a winning or losing strategy. If so, it’s more a matter of timing than of being brutal, at least for hypothetical perfect players. Whether humans can do much here, I wouldn’t know.