All dark leafy greens contain oxalic acid. I'm surprised your vet didn't know about spinach; it's one of the ones that is usually unfairly vilified, while the same person will go on to suggest kale, which also is high in oxalates. Even some surprising sources like almonds, beets, and raspberries show up on lists of high oxalic acid foods.
Oxalates bind with minerals, especially calcium, to prevent the body from absorbing it. This obviously sucks if you are trying to absorb calcium, but it can also lead to kidney stones.
I think it is best to alternate as they all have different profiles.
Here are a couple of (people) articles, but I know even in reptiles like bearded dragons, this subject comes up a lot.
This is an article about oxalate and its health effects. Oxalate is an antinutrient found in many plants and has been linked with some health problems.
www.healthline.com
Do you love dark leafy greens? I love them, too. But did you know that you have to steam dark leafy greens before you eat them? Here's the reason why.
traditionalcookingschool.com