While the options are a bit more limited than older formats, Pioneer does still offer a few strictly better Shocks to
consider, namely
Wild Slash and
Play with Fire.
Wild Slash doesn't help too much, but
Play with Fire is always going to be better than
Shock. In your case, however, I'd advise
Wizard's Lightning. It doesn't work quite as well with
Dreadhorde Arcanist, but I think that extra damage will matter more often. If not,
Play with Fire is still a better pick than
Shock.
The idea seems really nice, I love the idea of stacking multiple
Harmonic Prodigy and
Naban, Dean of Iteration triggers to kill someone with cheap commons.
Panharmonicon is a bit pricey manawise and moneywise, and might be unnecessary overkill, but it works with
the general theme.
Unfortunately, because the Modern Horizons cards are Modern-(and other Eternal Formats)-only, it isn't a Pioneer legal deck, though.