In summary, we systematically explore the chemical space of the Cs-based quasi-2D HPs -defined by the spacer cations -that determines their phase complexity by implementing high-throughput automated experimental workflow. Through highthroughput PL spectroscopy and ML-based data analysis, several key features are observed. The RP-type molecular spacers with high molecular weights exhibit slow crystallization dynamics, thus manifesting the growth of 3D-like HP phases -more stable than the pure 3D CsPbI3 counterpart. DJ-type spacer exhibit stronger chemical interactions between the spacers and inorganic building blocks, which is maintained during the overall crystallization sequence -starting from top to bottom. The resulting quasi-2D HP matrices exhibit mitigated vertical phase inhomogeneities, although the films exhibit poor stability and the non-HP δ-CsPbI3 crystallites are involved. It is found that the molecular geometry of spacer plays a crucial role in realizing the stable quasi-2D HP lattices. The spacers with homocyclic, phenyl groups (PEA and DPA) exhibit a geometrically symmetric spacer-precursor interaction, which renders stable 3D-like HPs effectively protected by the aromatic rings. In contrast, the spacer with heterocyclic ring (ThEA) manifests asymmetric chemical interaction in the precursor state. This renders an upright molecular geometry at the 3D-like HP surface that cannot protect the lattice against O2 or H2O, thereby resulting in poor phase stability. These observations reveal the distinctive working principles of the spacer cations in the corresponding crystallization mechanisms, which have not been systematically identified so far. Thus, our findings provide new insights into the selection principles of molecular spacer for designing quasi-2D HPs.