I self-published my second novel 6 weeks ago - it is number 2 in a series.
Personally, writing the books was a walk in the park compared to the marketing. Like you, I spent over a year learning about book marketing - reading blogs, books, articles, watching videos, going on forums - David Gaughran and Chesson must think I'm stalking them :)
That was well over a year ago....and I'm none the wiser now I've tried marketing two books. It truly is a minefield.
I do feel that the day marketing agencies start charging based on their success rather than the number of hours work they do, we will all feel much more confident in paying for their services. But guess what? They are no more confident that they can work out Amazon's algorithms or guarantee sales from a pretty photo they put up for you on Instagram or post a tweet, than you are.
I HATE social media, but took a deep breath and prepared to prostitute myself on Instagram. I still don't know whether what I have done is responsible for sales - and that's the problem with marketing more often than not. It really is tough to work out which of your marketing methods have worked/are working.
I don't wish to come across in a negative way but sadly I do think there are soooo many people out there claiming they will deliver a brilliant marketing campaign for you - but is that what you want? I want them to deliver me brilliant sales...and they steadfastly refuse to do that.
I have learned a bit by trial and error and it's not a bad way to start. It's only by really understanding how the whole book buying and selling machine works globally that you could properly brief and monitor any agency you did decide to use anyway, otherwise, they can pull the wool over your eyes big time.
I truly wish you luck with whatever you choose to do.