When talking about deep thoughtfulness, the words that come to mind could range from contemplation to introspection. For a more solemn tone, words like solemnity or solemn thought could be used. One could also use introspectiveness if talking about an introspective nature. In a more positive light, the words mindfulness or consideration could be used to describe a profound level of thoughtfulness. Additionally, reflection or rumination could be used for someone who takes ample time to think and ponder on things deeply. Ultimately, there are many synonyms to describe deep thoughtfulness, and the choice of words often depends on the connotation and tone that the author or speaker wants to convey.