I have around 8 sites in different niches. The best approach is not to get attached to a niche and do your research. Make sure your niche has a good amount of traffic and the niche/services are paying a lot to the people doing the service. Plumbers and Electricians get paid a lot but the issue is they are too busy and don't need to buy leads or rent a site. I will give you a niche I'm looking at building a site for which is solar panel cleaning most of the time the cost to clean solar panels is around $300+ I'm in Australia so could be less in other countries.
Semantic Mastery is good for rank and rent videos, however, don't blindly follow Semantic Mastery or any group. There are lots of ways to rank and some are better than others the trick is to cut through the BS unfortunately cutting through the BS will take time.
You need to look at your sites like a tree it starts small but over time if you look after the tree it will grow big same as websites.