Question

How does the brightness of a bulb tell you about the energy of electrons passing through it?

Answer 1

The brightness of the bulb doesn't tell you anything about the energy of the
electrons passing through it.

It tells you how much total energy is lost by the total flow of electrons through
the bulb.  You can't tell what part of that is due to a large or small number of
electrons, or what part of it is due to each electron losing a large or small amount
of energy on the way through.