If you need waterproof trousers, that means it's raining: you don't want them to be awkward to put on in a hurry. When it's not raining, you don't want them to be the only thing that fits in your bag.
So, let us present the Venture Half Zip Pants: light, packable, simple to get on, and very reliably waterproof.
1/2 length leg zips mean you can get them on easily over big boots and, when they're in your bag, they tuck neatly into their own pocket so as not to coil around the rest of your stuff. Everything waterproofs should be.