There is the case of an electron interacting with a positron resulting in two gamma rays, but positrons are rare being an antimatter particle.
In the usual situation an electron is associated with a group of neutrons and protons and as it settles in with these it gives off photons of very specific wavelengths. It seldom falls into the nucleus giving up all of it energy.
Even when it does the proton that turns to a neutron would have gained mass. (I don't know too much about this but will look it up soon.)
I have never heard of unravelling an electron all the way down to nothing.
For that is what your equation seems to be suggesting to me. To keep on stripping the electron of its mass you would need to keep putting it nearer the largest possible nucleus. What is the heaviest atom possible? Do they draw the electron into the nucleus and become unstable?