Lanthanide-doped nanoparticles (LnNPs) are promising for advanced photonic applications due to their unique optical properties. However, their practical implementation is hindered by surface quenching and weak absorption. Surface passivation through core–shell architectures is effective in mitigating quenching. However, it creates a fundamental trade-off by impeding molecular sensitization via energy transfer (ET) in the organic–inorganic hybrid systems. Here, we investigate this trade-off by fabricating core–shell LnNPs with precisely controlled shell thicknesses ranging from 0.8 to 3.0 nm. Surface passivation yields enhancements in 290-fold upconversion intensity and 25-fold downshifting intensity. Using 9-anthracenecarboxylic acid, we demonstrate that ET efficiency exhibits a nonmonotonic dependence on the shell thickness, with optimal performance achieved at a shell thickness of ∼0.8 nm. Through steady-state and time-resolved spectroscopic studies, we elucidate the complex ET dynamics. Our findings reveal the optimal shell thickness and answer whether no shell is the best in this nanohybrid system.