Superextremal galactic Reissner-Noström blackholes

To celebrate 10 upvotes but no attempt at answers of this 8-year old question regarding extremality of Reissner-Noström blackholes, Let’s formulate a physically feasible scenario for driving a galactic black hole well over extremality via charge injection

As it is estimated in the original question, a black hole with a little above 100 solar masses should be stable against Schwinger vacuum breakdown, meaning that adding enough charge to make the black hole extremal does not require to exceed the Schwinger vacuum breakdown electric field near the horizon

But the electric field scales inversely proportional to the BH mass squared, which means that for a galactic black hole (well above the order of millions of solar masses) the electric field can be well below what a physically realisable ion/electron source needs to put to throw charge into the BH from the asymptotic flat region.

If all assumptions are correct, then this scenario should enable to exceed extremality and expose singularities to the asymptotic region, which would led to all sort of unexpected phenomena

The only assumption I am unsure is that the Schwinger breakdown electric field is not affected by the gravitational field, but if there was some empirical squared relationship between surrounding mass and vacuum breakdown, the above conclusion would be avoidable

Is this conclusion avoidable? If not, could there be a theoretical foundation (hopefully within known semi-classical theory) to a vacuum breakdown relationship like:

$$ E_{text{Schwinger}} propto M^{2} $$