Hi Giorgio
According to your argument even Quantum Mechanics is not ab-initio model because it is based on assumptions such as absolute square of wavefunction tells us about probability of finding the partical in a certain state.
I think, every theory makes some kind of assumptions to explain observations. Yes, classical electromagnetism uses newton’s laws to explain motion of particles but without classical electromagnetism, it is not possible to describe motion of charged particle. There isn’t any more fundamental theory to explain motion of charged particle. Therefore classical electromagnetism should also be an ab-initio model.
Let me know if you agree with me.