Is there way to set up trade routes to maintain a certain goods level? For example, if I have three islands where I might want to build, is it possible to set up a route to ensure that a certain island is always "topped up" to, say, 30 building modules but no more?
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.