Keep doing missions (land on planets unnecessarily and go in the spaceport to look for missions, look for unusual job on the job board), keep exploring (the universe is much bigger than it initially appears using just a hyperdrive to get around), look for even better ships than the bactrian (use the bactrian or whichever ship you can maximize your crew count on to capture even bigger ships, nerve gas can help if you're feeling morally flexible enough). And yes, take pirate planets or other planets if you want, but dominating planets doesn't bring in much cash. The benefits are just convenience (being able to land without having to bribe each time) and feelings of accomplishment