A Pali dictionary will likely be of more use than a thesaurus. A thesaurus provides synonyms that support a variety of expression. However, the Buddha taught in a very precise manner, using the exact same phrase over and over again for many many years. In mathematics, we don't look for synonyms of "line" since it is precisely defined.
However, the Buddha did in fact adapt his teachings to suit the varying needs of his audience. For example, the key phrase "root of suffering" or "dukkhassa mūlan" is found in several variations:
MN1:171.4: ‘Nandī dukkhassa mūlan’ti—
MN1:171.4: Because he has understood that relishing is the root of suffering,
MN66:17.1: Idha panudāyi, ekacco puggalo ‘upadhi dukkhassa mūlan’ti—
MN66:17.1: Take another person who, understanding that attachment is the root of suffering,
A thesaurus shows the connections between similar words, but the Buddha used the same simple, everyday words (e.g. "root") to reach all audiences. The subtlety of the teachings arises from the combination of these simple words into memorable, powerful phrases. This style of teaching naturally leads us to a search for phrases, not words. The words are indeed important, but only as atomic building blocks for the phrases.
And so we find that these phrases are key to understanding the suttas. It is these phrases, not individual words, that connect the suttas together in constellations of meaning.
If you would be interested in using phrases to explore the teachings, you might find it useful to look at voice.suttacentral.net. In the search box, simply type an English word and it will display key phrases that use that word. It's not a thesaurus, but it does help build mental maps in both English and Pali.