Forget "niche". Think "Audience"
"Niche", as a word, gives people a headache. Like so many computer and internet terms it's arcane and obtuse.
Yes, I know it's not "computer and internet terminology", it comes from good old fashioned marketing speak. Marketers talk about "market segment" and "market niche" etc. But every industry has its own "in" phrases. A kind of "shorthand". It serves two purposes. Makes "in house chat" rapid and concise. Makes outsiders look stupid.
Personally I prefer the term "audience". Everyone understands that a Black Sabbath audience won't respond to a Mozart pianist, and a Taylor Swift audience won't respond to Kiri Te Kanawa. Call them "niches" if you want because that's what they are. But it's not very helpful.
An audience is a group of people who want something, eg
- a regular bus service to their isolated village
- a nearby golf course
- a fluoride toothpaste
I would start out by thinking about an audience in these terms. Mentally get them into the local church hall and let them tell you what they want and why they want it. Write a journal of what they say. That's "keyword research".
Stick a title on the journal, and "boom" you have a "niche". And you have a book full of "keywords" that you can go do further research on.
Once you have answers from your audience, then you can stick on the "niche" and "keyword" and "keyword research" labels.