But seriously, I was actually looking the item two different ways: How I'd convert the item for my game, and how I'd convert it for a "faithful" conversion intended for sharing.
I don't think I've yet stated what I think you should do, but now I will.
S'mon's suggestion for the dust is excellent. It creates essentially the same effect as the original, and yet it's broader since some upcast spells affect more targets or have longer durations. It feels much more interesting that just more damage even though that is the primary effect. It's a neat use of leveraging 5e's rules to achieve the conversion.
That said, I think you ought to just convert both items to function as written: The rod is 1 use, magic gone, and tbe dust maximizes damage. The DMs using your conversion will know how to use the items in their own game.
But I also think you should use your own idea from earlier: Include a sidebar with a dust that work's as S'mon suggests (And offer varieties that increase the spell level by +2 or +3 too) , and a rod with charges that only suppresses magic (and varieties that do so for a month, a day, a minute).
Offer the conversion fathfully, but provide new related options!