Actually there is already a health bar for the ground defense, see the latest stellaris stream for that.It would be kinda cool if each "living" planet had a health bar, and if you fought too hard over it the planet would basically die and become barren. The habitability of the planet might also be affected if the bar gets too low and it then needs time to regenerate.
I would prefer the option to have 1 button so that it feels more dramatic, like "oh no, we started the process, we cannot stop it now!", and then you will have to repair the damage with terraforming.