I think this is a big part of the reason why Venom has evolved into a protagonist in his own right, since as you say, using him as a foil for Peter at a certain point just diminishes both characters, whereas you can do interesting things with either on their own.
Carnage is a bit trickier. He's such a hideously ghoulish character that you have to go pretty extreme in order for his premise to make sense, but once you go there, it's hard to justify him not getting killed. This said, when he's done right, he's such an outrageous, over the top slasher movie villain that he can be campy fun and you don't necessarily want to lose that. The best approach is just to use him sparingly when you have a really good idea and otherwise keep him out of sight and out of mind.