We exploit the high-symmetry persistent spin helix state obtained for similar Rashba and linear Dresselhaus interactions in a quantum well to revisit the weak localization problem within a perturbative approach in a Landau level formulation. We define the small parameter of the theory as the deviation from the symmetry state introduced by the mismatch of the linear terms and by the strength of the cubic Dresselhaus term. In the vicinity of the helix state, the SO field becomes uniaxial, offering a natural direction of spin quantization, thus defining the $z$ axis within the 2D plane. In contrast to previous theories, this reveals a full decoupling of the Cooperon triplet scattering modes as well as decoupled Landau levels, to lowest order in the small parameter. This makes it possible to derive a closed-form expression for the weak localization magnetoconductivity, thus providing a new paradigm of localization in the weakly-broken spin symmetry regime. We perform quantum transport experiments in GaAs quantum wells, finding very good agreement with the new theory. We present a reliable two-step method to extract the SO and transport parameters from fits of the new expression, obtaining excellent agreement with recent experiments. This is an important step towards engineering and controlling the spin-orbit interaction as a powerful resource in emerging quantum technologies.