Sn2+ instability is the critical bottleneck preventing the widespread implementation of lead-free tin-halide perovskites. While quasi-two-dimensional (Q2D) Dion-Jacobson (DJ) structures offer the leading stability solution, the field's reliance on empirical, application-independent trial-and-error for molecular selection has hampered progress. Herein, we progress beyond these limitations, presenting a high-throughput first-principles study of a set of chemically diverse diammonium spacers to establish a predictive design framework based on fundamental molecular characteristics. We unveil the following fundamental rules: nonpolar symmetry maximizes thermodynamic stability against decomposition; moderate steric bulk stabilizes polar spacers and promotes efficient packing; short spacers uniquely minimize out-of-plane hole effective masses by reducing electronic confinement; molecular polarity lifts electronic degeneracies near the CBM; aromatic cores could enhance electronic coupling near the band edges. These universally applicable principles provide the missing foundation for engineering stable, functional 2D DJ perovskites, accelerating their deployment in next-generation optoelectronics.