You might consider making a custom editor mode (FEdMode), which can spawn a toolbox of slate widgets in the top left panel, and process additional input / render additional things in the editor viewport.
This is how mesh paint, landscape, and BSP editing work, for example. Have a look at mesh painting or the tile map ed mode as a starting point (landscape is actually closer to your workflow RE: spawning additional actors, but it’s complex enough that it may not be a good place to start learning).
Note: If you go down this path, wherever possible, use methods directly on FEdMode or via GetModeManager (for selection/etc…) rather than going thru GEditor directly. In the distant future we’d like to break reliance on GEditor / a global selection state, and allow editor modes to be used inside of other editors such as the BP editor, whenever it makes sense. The tile map editor is taking the first steps in that direction and it can be used either directly in the level editor or in the standalone tile map asset editor.
Cheers,