If you are suffering from "keyword search burnout", here are other methods for finding material. It's a good idea to regularly employ these tactics as part of your search process.
Mine the bibliographies of the resources you have already found to identify other relevant material.
Similar to that, use the "cited by' and "related articles" links in Google Scholar. Look up the better articles you already have and see if they can lead you to additional literature.
Search by author. If you know the name of an author who has written extensively in your field, use Discovery or Google Scholar to see if you can locate more of his/her works. Both tools have an Advanced Search screen which allow you to focus in this way.
Check out a specialized database for literature specific to your field. For help finding a specialised database, consult our research guides.
Search for theses and dissertations on your topic. Theses and dissertations usually have extensive bibliographies that you can try mining. The Library has a subscription to ProQuest Dissertations & Theses, which provides access to most North American theses and dissertations published after 1997.