Mixed-cation engineering has been widely adopted in metal halide perovskites to improve film quality and stability, but its impact on optical gain in perovskite lasers remains unexplored. Here, we investigate the amplified spontaneous emission (ASE) property of A-site mixed perovskite thin films by systematically varying cation composition and correlating ASE thresholds with material and optical properties. To suppress surface-related optical losses, a trimming process was employed to remove defective top regions, leading to reduced ASE thresholds, with further stability enhancement achieved by incorporating a two-dimensional material. Photonic optimization via slab waveguide analysis identified an optimal film thickness (~54 nm), while a symmetric waveguide structure using a PMMA overlayer enabled additional threshold reduction. These results demonstrate that combined material, surface, and photonic optimization effectively enhances optical amplification in perovskite thin films.