I often read that there is no known mechanism that prevents a charged black hole, if the charge is high enough compared to its irreducible mass, from forming a naked singularity. But I don't get how that is possible.

Imagine a sphere of charge $q$, with an “infinite” radius and “infinitesimal” mass. Imagine we start shrinking it. To do that we need to do work. That work will create mass, which will be added to the mass our sphere.

If we keep shrinking and shrinking, at some point we will reach a radius equal to

$$r = \sqrt{\frac{G q^2}{2 \pi \epsilon_0 c^4}}$$

At that point, the mass that we will have piled up through our shrinking will be equal to

$$m = \sqrt{\frac{q^2}{8 \pi \epsilon_0 G}}$$

Is that not a black hole with an event horizon? What should I do to create a naked singularity?

P.S. For heuristic purposes I reduced to $1$ the $\frac{5}{3}$ factor that we normally use to calculate the energy of a charged sphere.