I don`t want my client pays me for nothing

First off, I`m new at doing SEO freelance. Still learning a lot! So, I have a client who is now ranking pretty well for their keywords. 3rd organic on first page. What else can I do to improve this? Or what can I do to keep the client paying? I`ve done most of the on page SEO. I`m working on better backlinks. And Google reviews are coming. I`m not real sure on how to retain this client. I want to make sure I am working for it. I don`t just want to have them pay me for nothing. Any and all feedback is greatly appreciated!