Step down variable high voltage DC input to low voltage DC output
Take a look at how off-the-line switch mode power supplies work. The very first thing they do is rectify the AC input to DC, using a bridge rectifier. You could literally use such a supply that's rated for 230V AC and connect it to your DC source, bearing in mind that two out of the four diodes in the bridge will be carrying the load. With your modest current / power requirements this should not be an issue.
Using an existing AC-DC switchmode power supply with DC input not only solves your step-down problem, it also isolates your low-voltage side from the dangerously high DC.