There are many synonyms for the phrase "weakly sentimental". Some possible options include overly emotional, maudlin, saccharine, soppy, mushy, cloying, syrupy, or sentimental. These words all suggest an excessive or overdone sentimentality that may come across as insincere or lacking in depth. Other related terms might include kitschy, cheesy, or cliched, which describe a similar sentimentality that relies on tired tropes or overused conventions. Ultimately, the choice of synonym will depend on the specific context and tone of the original phrase, as well as the desired effect or impression that the writer or speaker wishes to convey.