A full fare GE - Lausanne train ticket would be 29.- each way. So the first and most obvious step for saving money would be to get a Half Fare Travelcard for 190.-, this will allow to buy ½ fare train tickets between GE and Lausanne and also reduced fare Mobilis tickets within Lausanne. Note that this is a subscription-type contract which would auto-renew after a year, so don’t forget to cancel it again in due time when necessary.
For your return trips on fridays you could consider buying Supersaver tickets up to 1 day in advance (online only) at approx. 30% discount. But those Supersavers are strictly tied to the particular train you bought it for and are not refundable in case you miss it, even if your incoming flight is late. That’s why those make only sense on fridays in your situation.
Try to estimate how much you’ll spend on SBB and Mobilis tickets until end of year with the measures mentioned above. If that amount exceeds 800.-, additionally a Half Fare Plus 1000 prepaid credit could make sense for some additional savings.