Abstract: Rising global temperatures alongside increasing energy demand highlight the imperative for sustainable and energy-efficient refrigeration technologies. Magnetic refrigeration, based on the magnetocaloric effect (MCE), presents a compelling solid-state alternative to traditional vapor-compression systems. However, many high-performance magnetocaloric materials rely on critical elements such as rare earths, cobalt and germanium. Despite extensive compositional flexibility, high-entropy alloys (HEAs) have predominantly been investigated in equiatomic compositions incorporating significant quantities of highly critical elements to achieve large MCE or mixing rare-earth elements in majority proportions that only yield moderate MCE values, thereby failing to address issues of material criticality. In this study, we present a criticality-aware design strategy for the MnNiSi-HEA system, exemplifying a prototype of the latest third-generation HEAs. Various substitutional approaches were evaluated to achieve the coupling between magnetic and structural transitions. The most effective pathway, identified through the co-substitution of Fe and Cu reduces the structural transition temperature by over 900 K relative to MnNiSi while preserving the ferromagnetic characteristics of the low-temperature phase, successfully inducing a first-order magnetostructural transformation near room temperature. The resulting alloys, Mn_0.5Fe_0.5Ni_1-xCu_xSi, exhibit coupled transitions spanning more than 100 K and demonstrate the highest MCE reported to date among HEAs free of cobalt, germanium and rare-earth elements, outperforming previous records by 360%. Complementary density functional theory calculations confirm the stability of the orthorhombic and hexagonal phases. Predictions of lattice entropy change closely match calorimetric measurements. This study establishes a new benchmark for low-criticality magnetocaloric HEAs, underscoring that optimal functional performance and sustainable material development can be achieved concomitantly. The proposed design methodology offers a valuable framework for advancing resource-resilient solid-state cooling materials and underscores the potential of HEAs as a platform for sustainable functional materials.