We consider a relay-aided massive multi-input multi-output (mMIMO) system where a base station (BS) serves various users via multiple relays by employing non-orthogonal multiple access (NOMA). We practically model this system by considering spatially-correlated Rician-faded channels, and channel estimation errors both at the BS and at the users, which consequently perform imperfect successive interference cancellation (SIC). We consider these two artifacts, and derive a lower bound on the spectral efficiency (SE) of this multi-relay NOMA system, which is valid for arbitrary number of BS antennas. We crucially show that i) Rician-faded channels are immune to the errors caused by imperfect SIC at users; ii) high spatial correlation improves the spatial diversity among relays located close to BS and achieves better sum SE than its uncorrelated counterpart.