Where I live, people eat a lot of turnip, mustard, and collard greens. I handle these, as well as kohlrabi, rutabaga, beet, and chard greens, the same way. (Kale would no doubt be the same but I don't care for it.) They are fine when defrosted and reheated. The trick: fully cook them before freezing, and freeze the leftovers.
I love spinach, but beet and kohlrabi beat them out any day. Never had any luck growing spinach.
I have also frozen some of these when raw, and cooked them after freezing. Also just about as good as fresh. Trick for this: do not blanch. Clean and freeze raw; just wash/clean before freezing. But I have never done this and left them frozen longer than a month or so. Greens do not last long in my house.