Disclaimer right of the top…I’m not a pilot.
I do have a friend that’s an airline pilot on a Dash 8, and I asked the same of him with regards to how much of the route he plans, etc. His exact answer was…none of it. Operations sets up the routes, cost index, etc. and they’re saved in the FMC as saved routes. He just brings up the route in question and at that point he may have to make some adjustments but by and large, everything is programmed into the FMC prior to the flight.
Weight, fuel, SIDS, STARS, etc obviously are the variables that the pilots need to enter themselves.