  We investigate the twist angle dependence of spin-orbit coupling (SOC)
proximity effects and charge-to-spin conversion (CSC) in graphene/WSe$_2$
heterostructures from first principles. The CSC is shown to strongly depend on
the twist angle, with both the spin Hall and standard Rashba-Edelstein
efficiencies optimized at or near 30{\deg} twisting. Symmetry breaking due to
twisting also gives rise to an unconventional Rashba-Edelstein effect, with
electrically generated non-equilibrium spin densities possessing spins
collinear to the applied electric field. We further discuss how the carrier
doping concentration and band broadening control the crossover between the
Fermi-sea and -surface spin response, which reconciles the seemingly disparate
experimental observations of different CSC phenomena.
