If an agency offers you these services under the roof, there will not be instability for your business climbing on the SEO ranking. Its benefits will reveal itself as equality, common goals and low values. It will be easy for you to become a contact point for all services.
The most essential feature of any good digital marketing agency is that they should be transparent in their work. You should expect a detailed weekly or monthly report from them (which is also relevant for your business). Most SEO agencies are regularly distributing details of implementation strategies, their impact and timely details of the improvements.