There quite some people you can find to optimize your shopify store, but the most important things to optimize first are the basics of course and in some stuff you are somewhat limited in Shopify.
After the basics you can focus on things that you can scale such as link building - maybe Digital PR as well.
If you interested I can help you with those stuff. Shoot me an email email@example.com
Because you're looking for an SEO optimizer so I'd like to recommend you read this writing: How to choose the right Shopify SEO expert? As the title, this writing will tell you how to choose the right SEO expert as well as recommend some places where you can hire a good SEO optimizer.
Hope it helps!