Im currently taking the dregin down a peg or two and just taking away their most profitable planets but it seems that every time i conquer one of their planets it loses 1 planet quality turning 20's into 19's and etc. and i used traditional invasion tactics sooooooo basically wtf?
The drengin probably have a planet quailty ability that raises the level of their planet while they occupy it, so once you conquer it, it the level goes down because your planet quality ability is not at high.
And there are some mods with technologies/wonders/projects which increase your civilization PQ ability (don't mix it up with the local PQ ability due to Habitat improvement etc., these effect add up).
You can look up your abilities in the Domestic Pocilcy -> Stats area.
But be carefull, civilization PQ ability above 40 % will lead to anomalous scores in Metaverse, i.e. you can't submit your game.