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I just stumbled upon https://rhymebrain.com/en Given a word it returns a section that suggests rhymes for related words. They may not synonyms, but it is the only tool I have found so far.
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/47060 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/47060 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/