Yellowing or stunting, followed by early defoliation, are typically symptoms of Fusarium wilt, common to silk trees. Unfortunately, the disease is eventually fatal and there is nothing you can do with regards to treatment.

Since this tree is a listed invasive species for your area, removal should be considered a good thing and an opportunity to replant with a more desirable tree :-)

Hello. I was going to send a private message but I don't see a way to do that. I'm very curious to know what became of your tree. I hope it survived or at least went out in a blaze of flowering glory.