From what you’ve been saying, the G18 and G9 have separate BCCH already. Thus, your solution about changing the cell reselection offset is effective. However I’m afraid that you will reduce the coverage of G9 cells. Imagine an area which is covered only by the fringe of a G9 cells. The C2 criteria might drop below zero and therefore prevent someone to make a call. It might be safer to increase the cell reselection offet on G18 cells rather than decrease it on G9 celsl.
Don’t forget to set your handovers settings accordingly so the calls will stay on G18 cells (HO Margin, Ho thresholds)
You can also force your cell neighboring list to contain at least 2 or 3 G18 cells.
Finally, you can activate (or optimize) the force directed retry / directed retry in such a way that calls will go directly from a G9 SDCCH to a G18 TCH.