You need a fair amount of hydrogen gas to run a hydrocracker, which is basically what this is. Industrially you get the hydrogen from natgas. Yeah yeah you can get methane from the output of a hydrocracker and hydrogen from methan but depending on the process its net overall hydrogen negative (think about it, how are you going to split a long chain hydrocarbon that has two H ends into two little ones thus a net of 4 H ends without a continuous input of hydrogen?)

So yeah you're mostly right but never going to get away from maybe 5% to 10% capacity driving steam turbines to electrolyze water and run the (insecure, of course) SCADA and hotel load for the humans running the plant etc.

A likely plant would look like two nuke generating plants backing each other up and 18 nukes cooking algae into diesel, roughly.