WIY: Your preconceived notion seems to be that you can tell your 18yo kid what to do, and the question is how to do it in the most effective way.
I’d directly attack your preconceived notion. As a matter of fact, if there’s a question to be asked in this scenario it’s as follows:
My parents think they can control me. I’m 18 years old and my parents don’t understand the fact that I’m no longer a child. Additionally, they are considering making my financial dependence on them a tool through which to control me absolutely. How do I explain to them that at BEST they’re only driving me away temporarily, and if they’re not careful they can drive a wedge between us permanently?