Disclaimer : I am fairly new as well.

First of all, I want to tell you as a native English speaker that your English and your ability to write are excellent. As I was reading your question I never assumed that English was not your first language, after you stated that you were not a native in English I was surprised because your vocabulary and your writing ability are better than a good part of native English speakers.

In regards to your niche question, I cannot pick one for you however I can tell you that you MUST pick something that you enjoy and you can totally immerse yourself in so that you can become an "authority" in the niche you choose.

For example, someone who is not an animal lover would not have success in the pet niche

Someone who does not like football would most likely not find success in the football niche. etc etc

success in a niche directly correlates to your love for the subject and your knowledge about it. If it is something you love than you are much more likely to know everything there is to know about it and learning more about it will not feel like work.

I hope this has helped you figure out what to pick, just don't doubt your ability to compete against native English speakers!