Most big niches can be further niche down if you want to really micro target.
Say you're interested in pets. Or don't mind creating one since there's so many affiliate products for it.
Then you go down further, from pets, to dogs, to maybe dog food, then finally just dog food for specific breeds. Like you can create a site on the best dog food for german shepherds.
Same with other niches, from hobbies, to fishing, fishing lures, then fishing lures for specific types of fish, like fishing lures for black bass for example.
You get the idea. Of course you need to see the search volume for the keywords, but that's one way you find niches with low competition. By breaking down the bigger niches.