Looking for less sellers in a category while the niche being in good demand is important.
I am personally taking much more time with product research now by checking out the reviews more than anything, as it is much tougher to gather reviews these days.
I tend stay away from categories that exceed too many reviews, past the 300 mark with a few products (if that makes sense).
BUT, if there are other sellers with single number reviews in the same niche doing the same or similar revenue, then this would get me to look further into that niche by looking into competitors Amazon listing's quality and so on, paying close attention to price and image while doing so.
This may seem quite basic but this is exactly what I did when starting selling on Amazon 20-months ago, I think it's always best to return to the basics which have helped you gain success when going through certain things like product selection.
Depending on your comment, if the big player is sucking up all of the sales within the niche, why is this? be aware of any patents and look out for things they might be doing differently from their product creation and Amazon listing over their competitors.
Winning on Amazon isn'y all about having the most reviews these days, it is about creating a product that is unique and stand out from the others on market and offers greater help and value over the other products on market, this is long term thinking of your Amazon success too, which is something important to adopt.
Best of luck lunix, go do your thing! Enjoy the journey and learn as much as you can!