They tend to over complicate this easy step.
Here are some criteria I suggest you use for selecting a niche for your next Amazon Site.
The niche must have lots of people spending lots of money in it. Don't try to go for smaller niches that are not proven money makers. Choose a big niche and get your slice of the pie.
It's better to build one big Authority Site in a big profitable niche, rather than lots of smaller sites in tiny unproven niches.
The niche must have lots of Physical products that you can sell through Amazon, Shopify and other places. Can easily be checked by the number of reviews for products on Amazon.
Make sure some of the products that sell well are in the $200 to $400 range. I find those to be my best money makers.
The niche must also have lots of information products (books, Training courses, video courses etc.) being sold. You want to build a list on your site and info products are perfect for monetizing that list.
You can easily check that by going to the books section on Amazon and Clickbank. Do a search for your niche and see what there are.
You must have some sort of interest in the niche. I repeatedly make the mistake of going into niches that I could not care less about and although it is making me money, I find it a grind to work on the site and content etc. if I am not interested in the niche.
There you have it.
Choose a niche with the above criteria, build an Authority site around it drive traffic and start cashing in.