A class of septuple-atomic-layer two-dimensional (2D) materials, MA₂Z₄, is sought as an alternative to 2D hexagonal transition metal dichalcogenides in the fields of valleytronics and spintronics. In these materials, the structural symmetry can be varied by changing the stacking of its three parts in the monolayer. We use first-principles calculations to show that in the Janus monolayer WSiGeZ₄ (Z = N, P, As), Berry curvature and the Rashba effect are enhanced by modifying the stacking orders. The intrinsic electric field and composition of the d orbitals play a dominant role in determining these properties. Berry curvature is strengthened by up to 300% compared to its ground state through symmetry control, along with a significant increment in the Rashba coefficient. Moreover, monolayers WSiGeP₄ and WSiGeAs₄ have multiple valleys, implying another valley-dimension. The interesting spin-valley physics tunability in septuple-atomic-layer 2D materials suggests their exceptional potential for spintronic and valleytronic applications.