The Super Tau-Charm Facility (STCF), a next-generation electron-positron collider, adopts an on-axis swap-out injection scheme that demands high-charge electron bunches with exceptionally low emittance and minimal energy spread. This work presents a comprehensive and systematic beam dynamics design for the STCF electron injector. The bunching system, based on sub-harmonic bunchers and a traveling-wave buncher, is optimized using the Non-dominated Sorting Genetic Algorithm III (NSGA-III) to efficiently explore the high-dimensional parameter space. Start-to-end simulations incorporating space-charge forces, wakefield effects, and alignment errors are performed, with Beam-Based Alignment (BBA) applied to correct orbit deviations. The optimized injector delivers 8.5 nC bunches at 3.5 GeV with a normalized transverse emittance below 70 mm mrad and an RMS energy spread below 0.5%, satisfying the stringent STCF requirements. The high transport efficiency achieved in simulations alleviates the design pressure on the electron source, while the excellent beam quality inherently suppresses beam halo effects. By integrating systematic multi-objective optimization with high-fidelity simulations, the proposed design overcomes the intrinsic limitations of thermionic cathodes while capitalizing on their proven stability, robustness, and cost-effectiveness, offering an efficient engineering solution for high-charge beam delivery in next-generation colliders.