To be safe, I’d just give 2-3 times a week. In actuality, giving just a leaf every day, especially if paired with proper hydration and hay eating, likely won’t do much, but we are not nutritionists nor veterinarians, so I at least cannot say anything definitively.

That looks like a very nice variety in a salad!

To my knowledge, collards are not actually high in oxalate (none of the cabbage family veggies are), just calcium, plus the gas issues you noted. The main ones to worry about oxalate in are spinach, parsley, chard, and beet tops. So you wouldn’t want to feed two of those veggies in the same salad.

I usually aim to give at least 3 dif veggies per day, and only 1 of them is “allowed” to be high in calcium and/or oxalate. I also make sure that the high calcium and/or oxalate veg is not more than 1/3 of the total salad. Ideally I don’t give the same high oxalate veg multiple weeks in a row. So if I gave chard one week, I wouldn’t feed it the next week.

Dandelion is a really nutritious veggie for buns. I was feeding it daily for a while, but one of my buns is a senior and has gotten somewhat sensitive to high calcium veggies, so I don’t feed it often or in large amounts lately.