Improving the coercivity of high La/Ce content multi-main-phase (MMP) sintered magnets is crucial for promoting the use of La/Ce elements in permanent magnets. In this work, we used a novel strategy to accurately manipulate the chemical heterogeneity and grain boundary (GB) structures of MMP Nd-La-Ce-Fe-B sintered magnets, namely, by adjusting the composition of the LaCe-free alloys. We obtained an ultrahigh coercivity value of 13.75 kOe for the MMP-LaCe35 (LaCe accounted for 35 wt.% of total rare earth) magnet. This was attributed to the effect of Al-Cu-Ga element addition in LaCe-free alloys toward enhancing chemical heterogeneity (thicker Nd-rich shells) and optimizing the GB structures (continuous lamellar GBs). The former had a higher anisotropy field (H_A) value, which enhanced the nucleation field, while the latter had a lower Fe concentration, which improved the magnetic isolation effect. Based on the above mechanisms, a coercivity of 12.09 kOe was attained for the MMP-LaCe40 magnet produced utilizing an alloy with the same composition. In addition, reducing the Nd content while maintaining the addition of Al-Cu-Ga elements in the LaCe-free alloy proved that adding Al-Cu-Ga elements improved the GB phase wettability of the MMP magnets, thereby forming continuous lamellar GBs to enhance coercivity. Our findings revealed that the enhanced chemical heterogeneity also effectively improved the coercivity temperature stability of the MMP magnets. This study provided an effective and feasible strategy for microstructure manipulation to prepare ultrahigh coercivity MMP-sintered magnets with higher LaCe content, which is crucial for promoting the comprehensive utilization of rare earth resources.