Alright, so having mulled this over, a few thoughts. This weapon does not exist in D&D land so they are creating something new. If they want it, they should have to work for it. That first roll, I would call that a proof of concept. That as you point out will break on first contact.
This weapon has 5 components, it's new, it's all the damage types, and a shield. Before they can build a working weapon, there needs to be prototypes. Keep your DC 20, on a successful roll it doesn't break in combat but can't be used again. Also, with each iteration have the player describe what it looks like, and how it's changed. And, yes it had to be tested even if in the meta the player knows this iteration will fail.
After five prototypes, let them have the weapon. Most likely the bashing aspect will be from the Shield component so damage if fairly easy there, as for everything else, base it on the description, likely a d6 or d8 for slashing and piercing. If they want a larger die type, time to go back to the drawing board, and that means another prototyping roll, and maybe going two handed.
The key here is that smithing takes time. So this is likely to be something that happens between adventures. Build the prototype, go adventure, break it, come back, start the next iteration. Now it's a long term goal, but it is achievable and gives them something to sink time and gold into, as the prototyping shouldn't be cheap either.
Hope that helps.
Edit: When they describe each iteration, they should totally be drawing it, even if it is just stick figures, as we need to see this monstrosity.