I did some research on this a few years ago but never made the trip.
The main route south (through Patagonia) from Santiago is the Carretera Austral. I'm not sure about the route north as it's not visited by many cyclists, but it's a narrow coastal strip and there is probably only one route.
There are some journals on crazyguyonabike.com covering Patagonia.
https://www.outdoorroamers.com/cycle-the-carretera-austral/