Maximum inner-disc gas-phase C/O achieved for different parameters in smooth disc models with hydrocarbon formation around 1.0 M_⊙ TTSs (left) or 0.1 M_⊙ VLMSs (right). Darker green colours indicate more C-rich discs, while darker purple colours indicate more O-rich discs. We note that the colour bar is neither linear nor symmetric. The central circles show the maxima within the first 5 Myr, while the outer rings show the maxima across the full 10 Myr model run.