I eat collards all the time and do not find them bitter. I cut out the centre stalks, chop up the leaves, wash, and cook in 1" of salted, boiling water for just a few minutes. They really are cooking in the steam here as much as the water. I drain them and serve as is with a little salt.
But a great British recipe which actually originated to use up left- overs is to combine equal parts cooked collards with mashed potato and fry in olive oil until brown on both sides, rather like a large potato cake. It's called "Bubble and Squeak" and we love it in our family, I could never wait til left-overs day! It's delicious with a fried egg and some pickle.
I have had collards in restaurants cooked in garlic, so maybe some recipe books can help you refine the ingredients list for this one and cooking method.
I have never had kale, just don't like the way it looks. Can't help you there.
Clare