Have you considered killing the alianthus tree? I know loosing a big tree is a tough sell, but tree of heaven is one of our worst invasives in the eastern US. Every year that tree is producing wind-born seeds that mean it is going to be causing huge problems for anyone attempting to plant natives anywhere in your neighborhood and actively degrading any natural spaces in the area. You’d do the environment a lot more good just by getting rid of that tree than via anything you plant under it.

Also as an aside, it’s possible that the problem for your cup plant and asters is shade, not chemicals. There are a lot of species of aster so I can’t say for sure there, but cup plant is a prairie species that absolutely wants full sun.

Either way though, please, please, kill your tree of heaven.

Source: My wife does this for a living.