I built blog networks of expired domains and ranked for specific keywords... Unfortunately we failed pretty hard so far.
It's probably because blog networks are quickly found by Google and penalized. See here. Today, after Penguin and more, the goal is not to build links but to earn links.
You're falling into the old trap of thinking that "SEO" is a "bag of tricks" to get you to #1 for a set of keywords. Those tricks worked years ago, but thankfully, they no longer do. "SEO" is not something you do; "SEO" is what happens as the result of good web development and good marketing. "SEO" is a collection of best practices that help you to build an online brand that deserves to rank highly.
Take the search phrase "best London restaurants." Google does not want to list results based on keywords, content, or links. Google wants to list results based what the people living in London actually think are the best restaurants.
Today, the key to "SEO" success is to build an online brand through the following process:
1. Build a great website on a technical SEO level (fast-loading, optimized for mobile, etc.) with a wonderful user experience (UX).
2. Publish original, authoritative content on the topics that would interest your audience.
3. Use public relations and social media to promote both your website as a whole and the content on the website
4. Repeat steps two and three indefinitely, and traffic, links, rankings, and more will grow naturally over time.
It's not easy, and it takes time. But nothing good ever comes quickly.
What I guess I am saying: To be successful today, think less like an "SEO" and more like a "marketer." What makes you special and differentiates you from other similar websites? What is your brand? How will you position the website? What's your messaging? Those questions are more important to answer today than those about keywords and links.
Good luck!