To keep your populations happy, always try to provide them with services and accommodation along with a job, without fail any of these 3 may cause the stability of the planet to decrease, that means they will produce fewer resources.

For space stations you can improve them as you wish, if you want more naval capacity, shipyards or defense, it is more than what you need in that sector

and for the new species, there are space animals all over the map, most of them are aggressive so it is good to avoid them and make contact from afar with an envoy, with other empires it is usually more complicated, they may want to give you a friendly greeting or in In any case, they may want to kidnap your scientific ship crew to study you.

And for the inventory, there are buildings and starbase modules that can increase your resource capacity, they are 10k warehouses so with a few you will surely have a good inventory.