How many battles should it require to suppress a planet, crush it beneath your iron heels?!?! And hear the lamentation of their puny folks!
A planet with that starting domed city + two farms and a shopping centre + research lab for example would require only one battle at starting domed city to end in your victory to bring it under your control. Farms and a bunch of mall rats cannot offer up meaningful resistance to your stellar marines and orbital bombardment so they submit to your might after you take their domed city.
A planet with domed city + bunch of civilian infrastructures but five military hexs, and one of them also has underground caverns? That's a minimum of six battles to bring all of the resistance under your control. If its too much work for you, just use your local superiority in orbit and use those planetary bombardments. Be aware the planet quality will suffer in process for you're damaging the planet!
Find a sweet class 30+ planet? But all of it's hexs is covered in military hexs? That, my folks is a fortress world, not easy target to take at all. Best example of this can be found in a planet named Cadia.
Follow up to recommend that variations in starting places is strongly suggested to avoid boredom.