In addition to TubeBuddy's Keyword Search Tool, you can use YouTube or google's autofill. This is especially helpful if you are searching incognito or on a browser you don't normally use because you can began typing your niche topic and let the auto fill do the work for you.
Another great tool is called Answer the Public. You can type in one or two words and see a lot of different things. You get one or two free searches per day. What I do is use screen shot the different answers & put it in a Canva design so I can look back at them the next day. I do that for the auto fill too. I just did that yesterday for a brain storming session. Then if you have it narrowed down to specific topics, you can look to see how popular they are in search.