Lanthanide-doped NaYF4 nanoparticles are most frequently studied host materials for numerous biomedical applications. Although efficient upconversion can be obtained in fluoride nanomaterials and good homogeneity of size and morphology is achieved, they are not very predestined for extensive material optimization toward enhanced features and functions. Here, we study the impact of rare-earth metals RE = Y, Lu, La, and Gd ions within Yb3+/Er3+ codoped nanocrystalline REPO4 orthophosphates. The enhanced luminescent thermometry features were found to be in relation to the covalency of RE3+− O2− bonds being modulated by these optically inactive rare-earth ion substitutes. Up to 30% relative sensitivity enhancement was found (from ca. 3.0 to ca. 3.8%/K at −150 °C) by purposefully increasing the covalence of the RE3+−O2− bond. These studies form the basis for intentional optimization thermal couple-based luminescent thermometers such as Yb3+- Er3+ upconverting ratiometric thermometer.