Broad niche site SEO

Is it feasible to build a website based around a broad category (ex. tech) and then niche it down by categories with specific blog posts using much more targeted keywords to drive in traffic? Could a broad site be used as a long-term project to dive into multiple areas within the broad category and drive revenue or is it better to focus into the respective subcategories and make individual sites for each topic.

Does "down-niching" work as long as the content on each post is focused around much more specific keywords?

Depends on how much experience you have. If you know what you're doing it can work. If you're new, I HIGHLY recommend you start with a micro niche and learn the ropes.