There are many, many online tools for keyword (search term) research, and the first thing to think about is what you actually want to do. Is it to get ideas for articles? Is it to find out what related terms people are searching for? Is it just to check that your own list of targets are still good?
Your own Google Search Console account, with which you’ll be very familiar, is a great place to start, and to check the number of searches on the keywords you’re already ranking for. It can also provide some low-hanging fruit, especially by identifying those terms which are popular and where you’re on the second page. You could get a big traffic boost by working on these.
Google Keyword Planner (you’ll need a Google Ads account) might also be worth a look, particularly to discover interesting terms which you may have missed in Google Search Console.
For article ideas, I like Answer The Public, not least because it’s so beautifully done. This will give you endless ideas for questions your site could be asking and answering – have a play. There’s a lot in it.
There’s even more in Ubersuggest, which allows you to explore not only by keyword but also by domain. Try competitor sites as well as your own here. Again, you can learn a lot just by having a play, but be prepared to get lost down some rabbit holes!