Might be simpler just to accelerate a small un-crewed ship to approach the speed of light, and strike the planet with it as a kinetic weapon (causing an extinct

Might be simpler just to accelerate a small un-crewed ship to approach the speed of light, and strike the planet with it as a kinetic weapon (causing an extinction event but not enough to actually disintegrate the planet). Closely following behind can be a slower ship containing the colonizing party. Then there won't be a need to heat up the target planet.