In the presence of non-uniform gravity spacetime is curved and there are no global inertial frames. Inertial frames only cover regions that are small enough to neglect all tidal effects.
Local inertial reference frames do exist and are easy to identify. They are reference frames where all objects with accelerometers* that read zero move in straight lines with respect to the reference frame.
The local gravitational force, like all inertial forces, is not detected by accelerometers, so in a local inertial frame there is no gravitational force. Thus it is not possible to balance the gravitational force and the electrostatic force as described. In an inertial frame there will only be the electrostatic force, and therefore there is a real net force between the two. This will lead to greater acceleration apart than would normally occur due to the expansion.
Over cosmological distances spacetime is curved and there is no possibility of using an inertial frame. It is possible to write the equations of motion in some non-inertial frame. When you do so you will obtain some terms that can be interpreted as an inertial force, sometimes called a fictitious force. Generally, we do not bother to do that, and just attribute the so-called Christoffel symbols to the coordinate system rather than a force. But it can be done if desired, and even Einstein took that approach at least once.
*By “accelerometer” I refer to the 6 degree of freedom type that measures acceleration and rotation on all three axes.