Lead-halide perovskite nanocrystals (PNCs) exhibit exceptional optical properties with size- and dimension-tunable optical bandgaps, making them promising for diverse optoelectronic applications. The dimensionality of PNCs has been controlled by tuning the oleic acid (OA)/oleylamine (OLA) feed ratio in conventional injection-based synthesis. Although the emerging bimolecular nucleophilic substitution (S_N2)-based heat-up synthesis enables the production of high-quality and monodisperse PNCs, achieving dimensional tunability by altering the OA/OLA feed ratio remains challenging in this approach. Herein, we present an integrated strategy for controlling the dimensionality and surface properties of CsPbI₃-PNCs based on the S_N2-based heat-up method. Our approach focuses on the regulation of OLA protonation and oleylammonium (OAM⁺) lattice-forming behavior. Using chemical analysis, we identify the protonation routes of OLA in S_N2 reactions and demonstrate that adjusting the acid–base and S_N2 reactions involving OA and OLA can reduce OLA protonation and the subsequent formation of OAM⁺. Furthermore, by enhancing the competitive lattice-forming behavior of Cs⁺ over OAM⁺, we suppress excessive surface termination by OAM⁺. These approaches achieve dimensional tunability between two-dimensional (2D) nanoplatelets and three-dimensional (3D) nanocubes in the S_N2-based heat-up synthesis, along with subsequent spectral control of optical features. To independently optimize their surface passivation, we employ a post-synthetic passivation (PSP) strategy using OAM⁺–I⁻ pairs, which increases the photoluminescence quantum yield of PNCs from 71.9% to 81.7% by reducing surface defects.