A hybrid design of the Central Solenoid is under study within the conceptual design phase of the European DEMO fusion reactor. It is based on HTS conductors in the innermost high-field layers, and Nb3Sn and NbTi conductors for field values below 15 T. Since the resistance of the stainless steel jacket to mechanical fatigue drives the whole magnet design, alternative layouts of the cable-in-conduit conductors are aimed at decoupling the mechanical and hydraulic functions of the jacket itself. Thus, a reduced steel thickness, a smaller radial build of the coil and a larger magnetic flux are achievable. In this work, we analyse a new cable design based on filled conductors and tight He channels, both for the HTS and the LTS layers, and study their thermal-hydraulic performance by means of numerical simulations. After having introduced the geometrical and material details of the considered conductors, we describe the model and assumptions made for estimating the AC losses, assumed as the only heating source. Finally, we discuss the resulting minimum temperature margin of the conductors and provide useful insights for future activities on conductor development.