Organic-inorganic hybrid perovskite solar cells (PSCs) have emerged as a leading photovoltaic technology due to their exceptional power conversion efficiency (PCE) and low-cost fabrication process. However, the intrinsic thermal instability of organic cations, such as methylammonium (MA⁺) and formamidinium (FA⁺), necessitates their partial or complete substitution with inorganic cesium (Cs⁺) ions to enhance thermal robustness. While all-inorganic CsPbI₃ exhibits superior thermal stability, its susceptibility to moisture and phase instability limits its practical applicability. Moreover, the toxicity of lead (Pb) has driven interest in tin (Sn) as a more sustainable alternative. In this study, we investigate the incorporation of pseudo-halide thiocyanate anions (SCN⁻) as a crystallization modulator for two-step spin-coating preparation of Cs_0.1FA_0.9Pb_0.9Sn_0.1I₃ film, which promotes the formation of lead iodide coordination intermediates and lowering the energy barrier for perovskite crystal growth. By integrating Cs⁺ and Sn²⁺ into FAPbI₃ perovskites with SCN⁻ additives, the compositions, crystallinity, and grain interfaces of Cs_0.1FA_0.9Pb_0.9Sn_0.1I₃ film are well tuned, yielding a PCE of 21.34%. The resulting PSCs demonstrated superior long-term stability and enhanced thermal resistance, highlighting the immense potential of SCN⁻ mediated crystallization and tailored compositional engineering as effective strategies for the development of high-performance and thermally endurable PSCs.