<p>In this work, a set of effective semiconductor Maxwell–Bloch equations has been used to study the dynamics of a bound-to-continuum terahertz quantum cascade laser (QCL) under external optical feedback at both threshold and maximum operating currents. The used model is extended to include current-dependent input parameters obtained from density matrix transport simulations, enhancing physical accuracy and enabling device-specific modelling within the relevant current range for a specific fabricated QCL. Near threshold, external feedback shifts the emission from single-mode to multimode and, depending on the external cavity length, can produce different dynamical regimes. The feedback-induced reduction of the threshold current is demonstrated and quantified. At maximum operating current, the laser is inherently multimode, with external feedback producing different dynamical regimes. The model predicts high sensitivity of output power to variations in the external cavity. When operating with a few modes, the output remains interpretable for sensing applications, whereas highly multimode emission leads to noisy, less interpretable signals. This framework provides insight into self-mixing and mode competition phenomena, aiding in the identification of QCL designs that minimise multimode behaviour for reliable THz sensing and has a direct impact on the advancement of QCL-based sensing and communication applications.</p>