I actually just finished a Siam (aytuhaya) game recently. the trick is releasing and feeding vassals (returning cores to vassals gives a lot of karma, so does releasing small states that are very easy to vassalize). I got the white elephant achievement for owning and coring all of Indochina fairly quickly and i wasnt below -10 karma even once
I agree that the mechanic needs a bit more work as it seemed like little more than an additional strategic obstacle imo.