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Second near-infrared (NIR-II; 1000 to 1700 nanometers) aggregation-induced emission luminogens (AIEgens) hold great promise for imaging-guided precision tumor phototherapy yet are limited by the intrinsic trade-off between fluorescence brightness and photothermal performance. Existing design strategies often yield complex molecular structures while inadequately addressing this trade-off. Herein, we propose a "Minor Tweaks, Major Leaps" concept: Simple modification of the star NIR-II AIEgen 2TT-oC6B (less than 5% change in molecular weight) markedly improves both NIR-II fluorescence brightness and photothermal performance. The thus-obtained new NIR-II AIEgen, ST-CZ, ranks among leading phototheranostic agents, exhibiting a long emission maximum at 1082 nanometers, a high fluorescence quantum yield of 4.3%, and a great photothermal conversion efficiency of 69% in nanoparticles. Leveraging these superior properties, ST-CZ enables cerebrovascular imaging with an advanced resolution of 19.0 micrometers, and the derived multifunctional platform achieves tumor progression-associated vascular remodeling visualization and efficient metastatic tumor ablation via synergistic photothermal immunotherapy. This work provides a robust strategy for developing high-performance NIR-II AIEgens for advanced biomedical applications.