This study explores the influence of two different synthesis methods on the sintering behavior of three novel high-entropy oxides possibly suitable for thermal barrier applications: (Ce_0.2Zr_0.2Yb_0.2Er_0.2Nd_0.2)O_2-δ, (Ce_0.2Zr_0.2Yb_0.2Er_0.2La_0.2)O_2-δ, and (Ce_0.2Nd_0.2Yb_0.2Er_0.2La_0.2)₂O_3+δ. Rare-Earth-based High-Entropy Oxides (RE-HEOs), recently known for their exceptional thermal stability and compositional flexibility, have gained increasing attention as potential candidates for many advanced technological applications. Thus, our current work focuses on the specific effects of synthesis techniques, namely co-precipitation and hydrothermal treatment, on the entropy-driven stabilization, microstructure, electrochemical properties, and sintering behavior of three novel RE-HEOs. The obtained results reveal significant differences in terms of densification yield and of the obtaining of the designed entropy-stabilized single phase depending on the adopted synthesis route, underscoring the critical role of synthesis in optimizing RE-HEOs for near-future technological applications.