High-performance dielectric tunable materials with both high dielectric tunability and low dielectric loss are urgently needed for new-generation electronic tunable devices. In the present study, a new system, (Ba_0.675Sr_0.325)_1−xLa_xTi_1−xMn_xO₃ (x = 0.25%, 0.5%, 0.75%, and 1.0%), was designed. The acceptor dopant Mn was added to lower dielectric loss, while the donor dopant La was introduced to enhance dielectric tunability. The samples were prepared using the conventional solid-state (CS) reaction method and the semi-solution (SS) method. The experimental results showed that the morphology of the ceramics was optimized by further improving the processing procedure. Dense microstructures, homogeneous grains, and uniform dopant distributions could be achieved successfully by the semi-solution method. Moreover, a significant enhancement in the tunable properties was realized owing to the improved microstructure mentioned above. The optimum tunable properties occurred in the samples prepared by the semi-solution method at x = 0.75%, with a high dielectric tunability of 85.0%, a low dielectric loss of 0.0011, and an excellent figure of merit (FOM) of 773. The tunable properties of (Ba,Sr)TiO₃ (BST) ceramics were even superior to those of lead-based materials, with an FOM of approximately 700. All the results suggested that the semi-solution method rendered BST ceramics more promising for applications in tunable devices.