Dimensionality strongly affects elementary excitations in correlated quantum materials, yet its impact on extreme nonlinear optical responses remains largely unexplored. Here, we combine high-harmonic generation (HHG) experiments on quasi-one-dimensional SrCuO₂ and quasi-two-dimensional Pr₂CuO₄ with nonequilibrium simulations of Hubbard models. We find a pronounced dimensional contrast: SrCuO₂ exhibits a robust plateau-like high-harmonic spectrum with weak temperature dependence, whereas Pr₂CuO₄ shows a monotonic decrease in harmonic yield and strong thermal suppression, especially at higher harmonics. The simulations qualitatively reproduce these trends and identify dimensionality-dependent doublon–holon dephasing, governed by spin–charge coupling, as their microscopic origin. These results establish dimensionality and spin–charge coupling as key control parameters for extreme nonlinear optical responses in correlated insulators.