while it is an interesting idea, I don't like the idea of population being the modviation, look people deal 20 damage to a planet (most planets) for 5 energy, so 5 energy for 20 ship kills "tax" is not really anything to care about, I think the downsides need to fit the upsides, All yours would do is instead of a invade foot ball it would be an invade foot ball thats also an population sink. that when you become the new owner are expected ot use a cloning pod on

Here are some draw backs that are more realistic, note I am not saying ALL of them but likely some combination of them

Draw back, Church's

your people demand that all planets have churches to honor your god, all planets lose 40% of their space, No! terrible idea!

Draw back Lack of faith

Many planets as they urbanize begin to lose faith in their gods, after all if these planets have runes of old buildings, maybe god didn't put them their, Any planet that generates 200 artifact points an hour or more has a 1% each day of defecting and you LOSE the planet (everything even the planet on scan) Does this count invincible planets as well? And other planets people who have spent hundreds on in $$$ or £££?

Draw back WAR IS ALL

to maintain your gods favor you must offer him constant combat and prof of such victory's, This amounts to each day inorder to use any of your god ability/tech you must pay 50 badages any combination. (if you do not you can use any of the gods tech/ability ) That seems unfair, 50 badges is a lot to pay each day, that's 18250 a year!

Disprove the GODS!

a collective knowledge base of your legions you can contribute research points to it and after collecting (some huge number?) you can disable any one god for 24 hours. (any followers of that god will NOT be able to attack your legion or members of your legion. ) No!