There are folks out there that pull off throwing together a bunch of topics, but it's rare.
Imagine you have two people you can talk to at a party. One guy is explaining, idk, how he just went on vacation to Europe and is describing the beauty, the culture, the sounds, etc.
The other guy quickly states "I've been there, it's beautiful! Did you know that Europe's been in a few wars? So has America! Did you know, in America, 100 acres of pizza are served in the US every day (true story)? I like pizza. Did you know pepperoni is the most popular pizza?"
With the first guy, you know what he's talking about and won't get lost in a well of confusion. The second guy would be exhausting trying to get any meaningful information from. Blogs work the same way. People will get frustrated if there are too many topics and will just assume you only know a little bit of a lot rather than a lot about a little bit lowering your influence and authority. Start with one topic and if you need to get other information out, wait until site #1 is established and smoothed out and start niche number two when you can dedicate some good time to it.