Unfortunately, you're doing it all wrong (in my opinion). Your chance of success is very low if you're just going to follow the crowds and do exactly what everyone else is doing.
I consider niche adsense sites to be the multi-level marketing equivalent of the internet business world.
These sites are a big problem for search engines because they're creating a bunch of spammy, low-quality pages that users have to keep sifting through to find the good stuff.
I've found that I can't rely on Google anymore for many of my searches due to these types of sites proliferating.
It's only a matter of time before we get better search technology that filters that stuff out (even possibly going back to a more human-filtered/curated search engine).
Instead of working on these random niche adsense sites, why don't you work on building a site around a topic you're actually interested in? Build great quality content, build an email list, build applications that help your target audience.
Choose a target market that's big and spends a lot of money, and create a niche within that market. It's easier to get a small piece of a large pie than a large piece of a small pie.