The use of metal halide perovskites in photocatalytic processes has been attempted because of their unique optical properties. In this work, for the first time, Pb-free Bi-based perovskites of the Cs3Bi2X9 type (X = Cl, Br, I, Cl/Br, Cl/I, Br/I) were synthesized and subjected to comprehensive morphological, structural, and surface analyses, and photocatalytic properties in the phenol degradation reaction were examined. Furthermore, the most promising Pb-free Bi-based Cs3Bi2X9 perovskites (i.e., structures that present an optimal combination of structural features and efficiency) were selected using a machine learning based virtual screening methodology and enriched with the C3N4 composite. The selected and designed Cs3Bi2 (Cl/Br)9 structures were in line with the “safe and sustainable by design” approach and applied for the first time in the aforementioned reaction, which allowed for approximately 84% efficiency under visible light. The main merit of this achievement was the optimum deposition of perovskite Cs3Bi2(Cl/Br)9 (5 wt%) on the C3N4 matrix, which resulted in approximately a two-fold increase in the apparent quantum efficiency of the composite compared with that of bare C3N4 (at an excitation wavelength of 430 nm).