Yep, collards are fantastic! I eat a cup and a half every morning with my eggs. They are very low in oxalates and very high in calcium, the highest of all greens. And don't discard the stems like you would on kale. They are very tasty, a little sweeter than the leaf. I'd forgo the spinach entirely as a calcium source as you won't get much from it but instead, you'll get a high dose of oxalates. Just have spinach on occasion if you like it but try to get your calcium elsewhere.