The antonyms for the phrase "more larkish" could include terms like serious, somber, restrained, or sedate. These words all suggest a sense of gravity or solemnity that is quite different from the playful and carefree connotations of larkish behavior. Other antonyms might include terms like responsible, staid, disciplined, or mature, which all highlight the importance of being focused and responsible rather than indulging in frivolous or extravagant behavior. Ultimately, the choice of antonym will depend on the specific context in which the phrase is being used, as well as the desired tone or mood of the communication.