TL;DRAllow the toolbar to have a single button represent all types of belts, splitters, inserters, pylons etc.
What ?I'm going to use belts for the example, but also applies to many other objects in the game.
Currently if you want to have yellow, red, and blue belts in the toolbar, they must each exist in their own slot.
Instead, you could have a 'belts slot' which represents all the different kinds of belts. This could be implemented something like a 'split button': https://stackoverflow.com/questions/159 ... t-winforms
When you click on it, the 'current' belt, is placed in your hand (like now).
If you hold down the mouse button on it (or click the little arrow to the right) then you get a flyout showing all the different types of belts. You can then select the belt you want and that becomes the 'current' belt. Now the icon for the button changes to represent the 'current coloured belt and when you want to change to a different belt type just select it from the flyout.