There are many different synonyms that can be used instead of "is insincere with." Some of the most common options include words like "duplicitous," which means deceitful or dishonest in conduct or speech, and "deceptive," which refers to someone who misleads or conceals the truth. Other potential synonyms could include "false," "untrustworthy," "disingenuous," "fake," or "hypocritical." Ultimately, the right synonym will depend on the specific context and the nuances of the behavior or communication that is being described. By using a more precise and descriptive term, however, you can help to convey a more accurate and nuanced understanding of the situation.