I agree its best to learn yourself, but it’s hard and there comes a point when you think “hmmm – how much further can I take this”. It’s tough, not least because there are no “answers”, only assumptions, although many are pretty accurate.
The problem with hiring an SEO is you don’t know what you are going to get. I would imagine a scenario where both SEO and Customer are both happy and feel they got/gave VFM at the end of a project is near impossible. Managing expectations in SEO is far from easy, because there are no guarantees.