Nomadic Interstellar travel

>> It is simple: people just have to colonize the trillions of
comets of the Oort Cloud. Then, someday, hundreds of
thousands of year in the future, another star will get close
enough to the Sun so that its cometary cloud will intersect with
the solar one. Then, people living at the Oort Cloud will be able
to migrate to the extrasolar comets making an
interplanetary-scale trip, no different from the journeys between
solar comets.

>> I saw this method as described by Carl Sagan in "Pale Blue
Dot", but I think that it was imagined by someone else long ago.

Find the book:
Finney, R. & Jones, E.M. (eds.)
"Interstellar Migration and the Human Experience" University of
California Press (1985).
An article called "Fastships or Nomads: 2 different roads to the
stars".
It will never be cheap to concetrate & focus the energies to
accelerate a ship to even reasonably high fractions of C.

The cometary could is a natural place for space colonies: plenty
of raw materials, and far richer in volatiles than those bodies
closer towards the Sun. Once you're 150+AU out there, the Sun
is only a particularly bright star, and others might even be
brighter.
Comets so far out are moving very slowly. a comet mining
colony matches to it, and starts mining it, and altering their
vampire ship's trajectory to overtake another, a few decades in
the future, when they'll be running out of volatiles, or need to bud
off a new colony. So what if the next comet their trajectories take
them to happens to be an interstellar wanderer passing
through? Relative velocities so far out are low.
It's not too extreme to imagine catching a ride which takes you a
few more km/sec away from the Sun, and so on for the centuries
they live out there. Eventually, they're on a trajectory which is
falling inward towards another star.

This is a natural evolution for space colonists, since they don't
think of looking for another planet; rather, comets are better
places to colonize. This means that any star will do.
Note the energy source: each person is given (arbitrary) 1MW
of power. This means that a colony of 500 needs about
30,000km radius mirrors (SWAG, I don't remember the article
precisely). With this, you get your power from the brightest star in
the sky, and it's not necessarily the closest.

Also note the population Finney & Jones specified: 25 "tribes"
of 25 individuals each. This has a historical basis, from the
Viking colonies, & the Greek colonies, & the polynesian
migrations across the Pacific. The book spoke of historical
instances, where a polynesian prince who was a younger son,
with no future on the home island, set out with a few followers
and their families, and a few hundred years later, there are
several hundred descendants, with no apparent genetic
damage. There was also an example of a European sailor who
took a few native women, and settled an island. 300 years later,
there are 1500 descendants, again with no apparent defects.

The colonists aren't primarily interstellar travellers. They're just
comet mining, space living people, who one day, find
themselves on the way... They're descendants of people who've
been mining asteroids & comets for centuries.

Note that they assume no new technologies, beyond
reasonably closed cycle systems, and asteroid mining (they
don't necessarily need 100% closed cycle -even if it's possible-
since they match with a new comet as needed).
They use huge mirrors to catch starlight, instead of fancy fusion
or antimatter (Impossible? Unknown.).
Propulsion is either magsail, solar thermal or electric, or
maybe even Dt/He3 implosion pulse.
The destination, and all the interstellar comets they meet on
the way are already mapped by fast flyby Wisp probes.
(Interferometry from multiple SSPS/radars on either side of the
Sun. Neptune L-4&5 points give us ~80AU baseline)

The Zen way to the stars: Living the journey is important, not
getting to the destination.