Good luck. There are tons of technology review sites out there already. If you want to succeed in that niche, then you have to stand out big and be extremely unique and have a site that's aesthetically pleasing.
I'm kind of in the same boat as you. Broad niche like say weight loss.
First target a subniche within like, so for the weight loss example it would be weight lifting (or something even more specific)
Rank up for that niche and once it's up, start targetting another subniche.
Keep repeating this and you'll have multiple subniches within your broad niche, allowing you to rank up for that broad niche.
It's a lot of work but it's needed if you're going to target something really broad.
Hope this helps