How do I maintain a constant amount of a good on an island via trade routes?

By right-clicking on goods in the warehouse or depot, you can set the minimum amount that is kept in stock.

Once you've set that, you can set a trade route to unload and then load from that port and it will keep the stock of that good at the amount you set as the minimum.

I did this in my sandbox game with just one "Building Materials" trade route. Using the Tycoon's ships that have 6 cargo holds, I had them set to unload then load the six types of building materials (building modules, tools, wood, concrete, glass, and steel) from each of my islands and had set a minimum on each island for these 6 goods. Set a handful of ships to the route and it worked pretty well.