Web dev looking to add SEO service to agency.

Hello, basically as the title says, I'm starting up a web agency, we develop mainly WP websites and we're looking to add SEO in our care plan along with hosting and maintenance. My question is is there any service offering a fixed monthly cost we could include in client's care plan that would handle 100% of the SEO in those website or any good reputable whitelabel agency. And also if you know any good courses about SEO and mind sharing them (can be free or paid), looking to expand on this domain aswell. Thanks!