This is basically B2B media, which has been around forever. We built a very profitable company about writing content that people in niche industries need to do their jobs, and then selling ads around it.
There are many factors that go into what industries we cover and who within that industry we write for, but the biggest one is probably that they all make buying decisions for stuff that costs a lot of money: https://www.industrydive.com/industries/
Just find any industry that has high margins for each marginal sale. That means there’s lots of marketing being spent.
Or a new product/industry/problem where it’s hard to find info.