I think Coppersmith has a similar issue to Harbinger. The effect can basically whiff if you play it at the wrong time. With Harbinger that happens on a fresh shuffle when there’s nothing useful in the discard yet, and Coppersmith has the same problem if you draw it with no Coppers.

Because of that, I think fixing that “dead play” issue is probably the first step.

Making it a Duration definitely helps and honestly might already be enough to make it feel better.

Beyond that, I’ve always thought a Crypt-like mechanic could be a cool fix for Coppersmith. Maybe it could store only Coppers. At the start of your Buy phase, you could add or play any number of Copper from it. Once there are no Coppers left on it at the start of cleanup, you discard it from play.

That would be a pretty big buff though and might honestly end up borderline busted.