It doesn't really make sense to say that "the delta V of departure is entirely given by the launcher which will reduce it by about 10 km/s". Just because a different stage is being used to accomplish the manoeuvre doesn't mean it reduces the delta-V. You will most likely launch to a parking orbit in LEO before inserting to a trans-Saturn orbit. Once you are in that parking orbit it makes no difference to consider the delta-V coming from an upper stage of the launch vehicle or coming from the spacecraft itself -- you are just distributing mass differently.
Of course overall you would want to use the launch vehicle to accomplish as much of the manoeuvre as possible so that you can ditch the empty mass to make future manoeuvres more efficient.
The question of how realistic a Hohmann transfer is for this scenario is really up to you. As mentioned in the comments, any mission to the outer planets is going to require gravity assists based on the current capabilities of human spacecraft. New improvements in rocket motors and propellants may make it possible to use a simpler approach in the future, but the more significant alternative is the use of ion thrusters (or perhaps solar sails) to achieve interplanetary transfer with a low, continuous thrust. Although using that approach completely changes how you look at the trajectory since we can't assume instantaneous velocity changes.
Since this is for a course project, I would suggest this strategy: use a Hohmann transfer anyway but acknowledge the limitations on delta-V and maybe even show sensitivity studies on how the maximum spacecraft mass changes based on different launchers (or maybe just vary the Isp for your propellant), as well as how the required delta-V (and/or Isp) changes based on a given spacecraft mass. Otherwise planning a completely realistic mission to Titan is impossible without gravity assists or a significantly smaller spacecraft.