- Are there any benefits to having a large population/high growth besides being able to load transports and colony ships more frequently
I can answer this one for you.
By default, a planet's Manufacturing, Research, and Income yields are based on the population of the planet. So for example, if you had 15 population on a planet, and 100% went to research, then you get 15 research points (plus some extra for techs and various things).
Then that number is multipled by your tech buildings and anything else that multiplies research. So when you buy a research lab that provides 45% more research, its 45% of that 15 number.
In this way, population is very important, its the main engine that drives your various resources.