Well, I seem to remember someone worked out the energy output in the standard laser turrets...something in the low hekatons range?
(10 hekatons=1 kiloton)
So...if you took over the space surrounding a planet after a set-piece battle, your warships would assume orbit and begin firing laser turrets in the way WWII bombers dropped carpet bombs...
Each bomb wasnt powerful enough to do a HUGE amount of devastation (i.e. a whole factory complex) but a ****load would take out the factory complex and most surrounding area. So, the fleet bombarding the planet with (relatively) low output laser batteries for a day or so would devastate population or industrial centers.
Plus, your Fenris and Leviathans have those cute little fusion mortars.
If THAT didnt work (resilient population, bombardment shelters, etc.), simply target planetary defenses, and send in the GTTs loaded with loyal GTA cannon fodder. Plus tanks, space and air craft, vehicles...etc. And have a nice little ground war with the ground-pounders.