That much greater fuel is needed to stop relativistically comes from a
sad lack of thoughtful incentive in all engineering and scientific
parties. How would a Stargate-level civilisation plan and power their
deathships? They would first move everyone toward the galactic core
and set and build up a new solar sustem amongst the accretion disc,
sacrificing some globular clusters to custom-make a Kerr black hole
gravity shield-pump around their new worlds just outside the event
horizon. Then they build another fleet in little outside time, make
all the antimatter they want from the pump, and go around the galaxy
pillaging the newly-made heavy fissiles everywhere from stellar
corpses. They /do not/ need yet more fuel to decelerate because they
would've devised a near-equipotential from their electrocolor fuel so
that their ships and their fuels travel in near-parabolic orbits about
each other: After a finite time, their fuel loops around with its
relativistic mass and brakes the ships to their initial speed: Their
momenta are regenerated, with low husteresis loss. For the last
affording, they may need to highjack a neutron star to polarise as a
momentum reservoir; by then, they have already experimented with
quantum gravity, seen its neutron excitation levels, and know what
dials to turn on their gravity pump to ration the star's heft-shot like
honey from a beehive. The neutronium ball may need to travel faster
than the ship though, for it to catch up as it rounds the event
horizon. All of this doesn't take into account the civilisation's
cheating the laws through another, but alike, route though.

-Aut
better life through new maths