Getting the keyword research and on-page optimization in place should be pretty basic work, unless you have tens of thousands of pages. This part of SEO is pretty commoditized, meaning that its a commodity and not a specialty item anymore On-page SEO hasn't changed much in the past 9+ years.

Where you'll spend most of your time and money is in link building. So the best "service" to hire, meaning you wouldn't do it in-house, is definitely link building. Even large successful SEO agencies have a hard time doing link building in-house and often outsource parts or all of the link building work for their clients. I know, I'm a link building vendor to many large and small digital agencies.