To what extent are the Daemon Primarchs “themselves” versus being a twisted “idea” of themselves brought about by Chaos?

A conversation late into the Heresy between Vulkan and Magnus comes to mind where Vulkan dismisses the Daemon Primarch by saying that “you’re not Magnus. You’re an impossible god’s idea of Magnus.”

This made me wonder about the extent to which the Chaos and Daemon Primarchs are truly “themselves”, entities that have been around for ten millennia and that share mental, physical, and psychological continuity with their respective selves. Additionally, I’m curious to what extent they are truly their own agents instead of being a very overt puppet of the Chaos gods and little more than an aspect of their deity made manifest.

I understand that the answer might vary widely between individual Primarchs, but I am curious as to what their individual cases might entail.