Here, (K0.44Na0.52Li0.04)(Nb0.9−xTa0.1Sbx)O3 [KNL−(N0.9−xTSx)] lead-free ceramics with potential technological applications are made through B-position substitution on the perovskite structure by Sb ions. This strategy generates crystal structure changes and stabilizes a tetragonal symmetry, T, giving place to an orthorhombic (O–T) phase boundary close to room temperature. Specifically, our results establish that an increase in relative volume fractions of the T on the O–T phase boundary produces an increase in functional properties. More relevantly, this improvement in properties is accompanied by controlling the grain size. Therefore, an increase in grain size promotes the increased domain size, favoring more domain wall motion and domain switching. Both effects result in an increase in the d33 coefficient. Finally, we believe this work helps to understand the factors that determine the improvement in the functional properties of the KNN–based piezoelectric ceramics.