  We investigate the effect of the twist angle on the proximity spin-orbit
coupling (SOC) in graphene/NbSe$_2$ heterostructures from first principles. The
low-energy Dirac bands of several different commensurate twisted supercells are
fitted to a model Hamiltonian, allowing us to study the twist-angle dependency
of the SOC in detail. We predict that the magnitude of the Rashba SOC can
triple, when going from $\Theta=0^\circ$ to $\Theta=30^\circ$ twist angle.
Furthermore, at a twist angle of $\Theta\approx23^\circ$ the in-plane spin
texture acquires a large radial component, corresponding to a Rashba angle of
up to $\Phi=25^\circ$. The twist-angle dependence of the extracted proximity
SOC is explained by analyzing the orbital decomposition of the Dirac states to
reveal with which NbSe$_2$ bands they hybridize strongest. Finally, we employ a
Kubo formula to evaluate the efficiency of conventional and unconventional
charge-to-spin conversion in the studied heterostructures.
