boy, juicing spinach, with all that acidity, must not be fun. Chard has similar amounts of oxalic acid. Why not try pea shoots? They grow all seasons. I plant them whenever I have a bed free for a month. Buy the peas for sprouting, not the expensive ones. You can jucie them until they are 10 inches tall. Hoe them under after five weeks. If you want fast growing and very spicy, mustard is a tremendous weed. Beet greens are a possibility, though they will not be available immediately. They might be amongst the best tasting. Kale tastes mild enough that I would try that too. And if you like bitter, chicory takes off really fast right now. if you are already picking carrots, their greens are a possibility (a bit sharp).
I only juice the collard stems in the winter, and that only thoroughly mixed with apples, celery, carrots or beets. Right now watermelon is the best juice.