Balancing output volume: Flow vs. specific constraints?

My initial excitement often leads to burnout when I attempt long-form journaling. To manage this, I am comparing 'unstructured flow' with 'micro-composition'—for example, writing three precise sentences about an urban setting rather than a full entry. Is this targeted focus more effective for long-term growth, or do you find the lack of breadth limiting? I want to avoid mental fatigue while maintaining nuance.