There is a surprisingly large literature of academic literature on Reddit an TD specifically, at least some of which mentions radicalisation, though I've yet to find specific bits.

https://scholar.google.com/scholar?q=reddit%20the_donald&btn...

Work elsewhere certainly expresses and substantiates that narrative: Kate Starbird, Renee DiResta, Samuel Wooley, Kara Swisher, Zynep Tufekci, among others.