Could you explain the gravity assist portion a bit more please? A gravity slingshot only works when you transition from a regime dominated by one body (sun for example) to a regime dominated by another body moving relative to it (Jupiter for many outbound probes).

I guess you could use a scheme similar to Cassini where you're doing multiple gravity assist swings off anything at all to get every last erg of thrust but my gut feeling is that you're not gaining much relative to the delta v needed for your complete voyage.

It would make for a fun app though. Design your own Interstellar Mission!