If electromagnetic waves cause disturbances in the Electric Field… what “is” in this E Field which photons Interact with?
I ask because in Vacuum, there are no electrons to excite. So what is “it” that's adding up in the E Field as a disturbance in wave propagation?
You are confused about electromagnetism. The thing is, there generaly (if we leave quantum mechanics out of it) is only electromagnetism, which can manifest itself as magnetic field, electric field, or usualy electromagnetic field depending on the reference frame you're observing it from.
What you need to know is that moving electric charges create a magnetic field that is propagating away from the charges at the speed of light.
Static electric charges create an electric field that is propagating away from them at the speed of light.
A changing magnetic field (accelerating charges) create an electric field around them.
Photons don't interact with electric field (under normal circumstances). They interact with charged particles like protons or electrons by exerting force on them.
You can Picture empty space as a space where there are many different fields which are 0 on average. When a photon, which is just an excitation in the electromagnetic field travels trough that space it is a wave of electrical and magnetic potential that is traveling trough the empty space. When it's at a certain point it increases a value of electric field in that point. Which you can actually measure. Now if you would to put an electron in that same location, the electric field (caused by photons) would interact with it, pushing it in some direction.
