Not to be too awkward but i dont understand how you can make a million year plan that will not be wildly out of date within 500 years. I would suggest that any interstellar craft we are capable of making in the next few thousand years will be made redundant by newer technology a few thousands later.

We need to at least get to a technological level where developing a "comfortable" interstellar probe is not a serious challenge. Either the system for propulsion has to be able to reach 80% of c, or the highest threshold speed where mass does not become a major problem which i think is around the mass doubling at 87%? Or if we cannot develop such propulsion system then we need to be able to build a very habitable craft which can carry generations of humans on long jouneys with a very high degree of safety and self-dependence.