Sometimes it comes down to hoping the AI does something stupid, like suicide into your starbase. It's definitely hard to do it consistently, sometimes AIs like to overbuild massively early.
It's not even close to comparable. Sure, you get ~2000 food from taking a colony. That's really not that much when you spent 1500 alloys to do it. The colony starts from 1 (working) pop like any other colony. That means a long time to get real productivity out of it. Aside from the influence cost and free outposts and mining stations, it's not significantly different from if the AI was never there and you just plopped down a colony. Totally different from getting 40 or so pops that can fairly quickly be altered to be perfectly happy workers and their planetary facilities swapped to make more alloys and stabilize your other economical problems.
Like I've done DS in a Grand Admiral Scaling game, and even when I control ~30 systems by 2250 I only have a total of ~350 of my own pops on those worlds. That's barely 10 pops per world, the majority of those worlds aren't that efficiently producing stuff and my total alloy production is around 75. In the same situation as a DA I'd have more like 1000 pops and 300 alloys. For a non-scaling Grand Admiral game DA is the only way you are going to snowball your fleet up to crush the AI across the galaxy in a wave of early wars. As a DS you conquer your first 1, maybe 2 empires that have a 2k-3k fleet, then you realize your next target has an 8k fleet and can only think "yeah, time to hang out doing nothing till I can catch up in 30 years".
