Gate geometry and bias asymmetry can be used to engineer spin dynamics in gate-defined Ge hole quantum dots by reshaping the confinement potential and driving transitions between distinct confinement regimes. Using a realistic three-dimensional device model, we show that these transitions strongly modify wave function localization, heavy-hole/light-hole mixing, and the effective vertical electric field, leading to pronounced $g$-factor modulation and geometry-induced dephasing sweet spots where the qubit becomes first-order insensitive to vertical electric-field fluctuations. We further find that phonon-induced spin relaxation exhibits a strong dependence on device size and bias, with $T_1$ following a magnetic-field scaling close to $B^{-9}$, consistent with Rashba-dominated heavy-hole spin dynamics. Our results establish gate pattern and bias design as practical tools for optimizing spin coherence in Ge hole-spin qubits.

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• Open quantum systems & decoherence
• Quantum information with solid state qubits
• Spin dynamics
• Spin optoelectronics
• Quantum wells
• Semiconductor compounds
• Spin qubits
• k dot p method