This work presents a six-level scheme terahertz (THz) quantum cascade laser (QCL) design in which the resonant-phonon (RP) and the scattering-assisted (SA) injection/extraction are combined within a single Al0.15Ga0.85As/GaAs based structure. By utilizing extra excited states for hybrid extraction/injection channels, this design minimizes the appearance of an intermediate negative differential resistance (NDR) before the lasing threshold. The final negative differential resistance is observed up to 260K and a high characteristic temperature of 259 K is measured. These observations imply very effective suppression of pre-threshold electrical instability and thermally activated leakage current. In addition, the impact of critical design parameters of this scheme is investigated.