Endohedral clusters inside metallofullerenes experience considerable inner strain when the size of the hosting cage is comparably small. This strain can be tuned in mixed-metal metallofullerenes by combining metals of different sizes. Here we demonstrate that the internal strain and mass can be used as variables to control Dy⋯Dy coupling and relaxation of magnetization in Dy-metallofullerenes. Mixed-metal nitride clusterfullerenes Dy_xY_3−xN@I_h-C₈₀ (x = 0-3) and Dy₂LaN@I_h-C₈₀ combining Dy with diamagnetic rare-earth elements, Y and La, were synthesized and characterized by single-crystal X-ray diffraction, SQUID magnetometry, ab initio calculations, and spectroscopic techniques. Dy_xY_3−xN clusters showed a planar structure, but the slightly larger size of Dy³⁺ in comparison with that of Y³⁺ resulted in increased elongation of the nitrogen thermal ellipsoid, showing enhancement of the out-of-plane vibrational amplitude. When Dy was combined with larger La, the Dy₂LaN cluster appeared strongly pyramidal with the distance between two nitrogen sites of 1.15(1) Å, whereas DyLa₂N@C₈₀ could not be obtained in a separable yield. Magnetic studies revealed that the relaxation of magnetization and blocking temperature of magnetization in the DyM₂N@C₈₀ series (M = Sc, Y, Lu) correlated with the mass of M, with DySc₂N@C₈₀ showing the fastest and DyLu₂N@C₈₀ the slowest relaxation. Ab initio calculations predicted very similar g-tensors for Dy³⁺ ground state pseudospin in all studied DyM₂N@C₈₀ molecules, suggesting that the variation in relaxation is caused by different vibrational spectra of these compounds. In the Dy₂MN@C₈₀ series (M = Sc, Y, La, Lu), the magnetic and hysteretic behavior was found to correlate with Dy⋯Dy coupling, which in turn appears to depend on the size of M³⁺. Across the Dy₂MN@C₈₀ series, the energy difference between ferromagnetic and antiferromagnetic states changes from 5.6 cm⁻¹ in Dy₂ScN@C₈₀ to 3.0 cm⁻¹ in Dy₂LuN@C₈₀, 1.0 cm⁻¹ in Dy₂YN@C₈₀, and −0.8 cm⁻¹ in Dy₂LaN@C₈₀. The coupling of Dy ions suppresses the zero-field quantum tunnelling of magnetization but opens new relaxation channels, making the relaxation rate dependent on the coupling strengths.