The Eurail Pass won't be good value for your trip for sure. First : trains from Interlaken Ost to Grindelwald, Lauterbrunnen and above are not included (25% discount though). And you do not have a lot of long-distance trains on your itinerary, apart from the TGV which would require a 29€ reservation with Eurail.
Buy an advance ticket for the TGV Lyria to Basel (sncf-connect.com). The earlier you buy the cheaper it will be. Weekends will be quite expensive, feel free to ask for advice mentioning travel date.
Then within Switzerland get the Swiss Half-fare card : 120 CHF for a month to get 50% discount on all public transportation. Already 100 CHF saved with Jungfraujoch !
Buy half-fare point-to-point tickets at any time through sbb.ch/SBB app or even at ticket machines or counters.
Be aware that Titlis is quite far from Lucerne (for Swiss standards) and that it closes early (before 5pm) so plan around that.
Don't buy tickets for Jungfraujoch or Titlis in advance : tickets cannot sell out and if the weather is bad you'll have lost hundreds of CHF. Have warm clothes and good shoes : I'd recommend going out of the main crowded building for a bit.
You probably won't ride trains in Greece.