Your torts may certainly eat that mix - including the spinach (assuming you keep your pets well hydrated).
However. "Baby" greens contain minimal amounts of nutrients and fiber. Mature greens are better (and are often more affordable by the pound). Buy one or two clumps of mature greens each week or so, and rotate different types of plants (dandelion, collards, kale - yes it is fine on occasion, curly endive, belgian endive, etc.).
Heads and bunches of greens can be rinsed and wrapped in wet paper towels to keep the leaves damp, but not wet, in the fridge. Loose bunches are easy to separate and wrap in layers. Heads should drain upside down before wrapping and storing to prevent rotting.
Go to the produce section of your food store where the "heads" and bunches of mature greens are and take a few clear photos of the greens and shelf labels if you need help getting started with what is available where you shop.
The warnings about spinach are out of date and not supported by science; it’s fine as part of a varied diet.
As mentioned above you would be better served by some of the more bitter lettuces: endive, escarole, radicchio, frisee. These will be down on the far end of the lettuce display; you’ll know you’re in the right place when you notice the prices! One of the nice things about the bitter chicories is that most of them last longer than the soft lettuces; we keep a head of frisee around for times we can’t get out for a more varied diet. Radicchio, endive, frisee don’t need to be shredded or broken up for storage, just cut a chunk off as needed. Radicchio and endive in particular act more like cabbage when it comes to storage.
If your store sells the bagged Ready Pac brand their Santa Barbara mix is exclusively bitter chicories so it’s a good one to buy.