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 wavefunction 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 T1 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.