The van-der-Waals antiferromagnetic topological insulator MnBi₂Te₄ is one of the few materials that realize the sought-after quantum anomalous Hall (QAH) state and quantized surface charge transport. To assess the relevance of its isostructural analog MnSb₂Te₄ as a potential QAH candidate, the roles of Mn/Sb site mixing and cationic vacancies need to be clarified. Recent findings have shown that non-stoichiometry in Mn_1±xSb_2∓xTe₄ is an efficient tuning knob to achieve a net spin-polarized state and to raise the magnetic ordering temperature well above that of MnBi₂Te₄. Here, we report the crystal structure, the bulk and the surface magnetism of two new Mn_1+xSb_2−xTe₄ samples: Mn_1.08Sb_1.92Te₄(x ≈ 0.1) with T_C = 44 K, and Mn_2.01Sb_1.19Te₄(x ≈ 1.0) with the record T_C = 58 K. We quantify the site mixing comprehensively by combining various structural probes on powders and single crystals, and then employ bulk, local (electron spin resonance), and spectroscopic (x-ray magnetic circular dichroism) probes to connect these insights to the magnetism of these materials. We demonstrate that Mn over-stoichiometry up to x = 1.0, in combination with a particular Mn/Sb intermixing pattern and the increasingly three-dimensional character of the magnetic order, push the T_C upwards. The tendency towards more robust ferromagnetism mediated by stronger interlayer exchange in Mn_1+xSb_2−xTe₄ upon increasing x is confirmed by bulk magnetometry and by a series of density-functional-theory calculations of model structures with varying intermixing.