Why not rework the piercing system entirely? More and more weapons seem to be working around the usual 10 tick piercing iframes (Blade Staff is one example, Zenith as well, Razorblade Typhoon only triggers piercing iframes for 6 ticks, Last Prism triggers it for 5 ticks).
Here's my idealistic approach to a reform. There are two types of piercing projectiles: those that linger (e.g. summons, Razorblade Typhoon, Last Prism), that I will hereby call the 1st type of piercing, and those that don't. (Jester Arrows fired off a bow, Retinazer summon's lasers, Meteor shots for the first hit), that I will call the 2nd type of piercing.
The 1st type of piercing should trigger iframes of a default of 10 for the single projectile that hit it. E.g. for the optic staff, 2 Spazmatisms are ramming into the enemy. The first one bites it; iframes of 10 ticks are triggered for it. The second bites it 5 ticks after the 1st one, but its damage still counts because the iframes only counted for the 1st spazmatism.
The 2nd type of piercing should completely forget about iframes, and instead make the entities it hits register 1 single hit, similarly to Luminite bullets, then be ignored by them forever. E.g. for the optic staff again, 2 Retinazers are firing at an enemy. The 1st one hits it, the projectile deals damage to the enemy for 1 tick, and no more registers are made after that. The 2nd retinazer hits it 5 ticks after the 1st, and its hit is registered in a similar manner. For a better example, let's pick the Meteor shot bullet. If you use it with the SDMG you're going to be able to hit one enemy a maximum of 6 times, despite its firing rate allowing a much higher output.
I'm pretty sure the Binding of Isaac works with piercing bullets that way. It feels more appropriate, too. 1.4 already made minions' attacks not block your own shots, why not generalize the situation?
tl;dr iframes shouldn't apply to every single type of damage