You need to focus on nailing that sub niche first.
It means that you need to optimize that sub niche.
Generate lots of quality backlinks.
Produce solid content, so you could offer a tremendous value to your target visitors.
Once you have that in the way, you can then start focusing on scaling up your operation.
Once you are ranking on your sub niche and it appears that your rankings will improve, then you need to start looking into tackling other sub niches.
Do all these with baby steps.
It's very difficult to run if you could barely crawl so focus on what is in front of you and just take it one step at a time.
Whatever you do, master each step.
Do not do anything in half.
Nail it down and think of scaling up.
That's how it works.
That's how you dominate a category.