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We combine phenomenological Landau-Devonshire (LD) theory and density functional theory (DFT) to investigate the effect of La doping on the structural and dynamic properties of La_xBi_1-xFeO₃ (LBFO). First, we compute the parameters of the LD potential via an expansion relative to the reference cubic phase as a function of all the relevant degrees of freedom: polarization, antiphase rotations of oxygen octahedra, and strain. For several concentrations of La (x = 0, 1/8 and 1/4), we determine the LD coefficients by requiring that we accurately reproduce the DFT energies and the distortion amplitudes for a set of relevant LBFO polymorphs. Then, we utilize this LD potential in Landau-Khalatnikov simulations of ferroelectric switching in LBFO, to predict how La doping affects the trajectories and the electric field thresholds of switching.