First thing, I’d probably solve practical fusion power and distribute that info worldwide… make it as available as the knowledge for building a fission reactor is today. Then, I’d set out away from humanity, into space. I’d start by visiting the nearest several dozen solar systems to begin the terraforming process on a handful of planets. Once started, I’d signal back to Earth that I’d done this and that the planets would be ready for settlement in about two hundred years. I would then go on to build an Expanse type “ring space” pocket dimension with wormholes connecting all these star systems, but put the “Sol gate” to this network in orbit of Saturn. Hopefully this would limit humanity’s settlement rush to some unmanned probes for the first dozen or so years, possibly only seeing manned missions in about a century, but not make it impossible that humans could spread amongst the stars using the gates. Once most of the systems wirh gates are settled, I’d return to gift the knowledge of gate building along with several dozen more star systems all with ready terraformed planets.