After reading around the forums I've concluded that this is the fault of the perfidious windows update. I updated my Nvidia graphics driver, my AMD chipset drivers, uninstalled the game, deleted the CK3 folder in my documents, reinstalled the game, and didn't encounter the freeze as of yet. Previously I had disabled fullscreen optimizations and made the registry edit suggested in another thread, these did not work.
EDIT: Happened again on the second boot when I tried to enable mods. Disabled the mods and it boots again. Tried to boot a third time with no mods and it hangs again. Very bizarre.