First, it seems that 600sq ft would be more than enough room for you and the cats along with their litter boxes. If you really think you can't lay out the area so that it can accommodate the litter boxes, then consider adding a small room, or large closet, to the garage instead of buying the shed.
I really can't see the cats tromping out to the shed in all kinds of weather just to use the litter boxes and I have no doubt the one box you keep in the garage will end up being used all the time.
Plus, are you prepared for the insects, mice and who know what that the cats will be bringing in from the run?
My cat has a run from my BR to a large kennel, but it is all on a covered deck. Even with that set up I've had to catch all the little critters mentioned above after my cat brings them in during the night! But, at least there are no more mice after I removed her heated bed from the kennel!

The other problem with the shed is that you'll have to partition it off if you intend to also use it for storage since it's very likely it will become an escape route for the cats when someone opens, or forgets to close, the shed door.

Good points. My cats have gone outside at various times and never brought trophies back inside, luckily. And it would have to be covered.

We are limited by local codes as far as what we can build. The garage is already at the limit for an accessory building, and I don't know if I will be able to add on to it - but sheds aren't regulated, since they are considered to not be permanent structures.

I know 600 s.f. is a lot of room in one way, but I need 4 cat boxes, and even in my much larger house, they are smelly even when cleaned daily, and in the way.

Check Pinterest for "Catio" and "Cat run".

If it's just to get them to the litterboxes, you need two cat doors and 5' chicken wire. Make sort of a hoop-house between the two buildings.

Plans for covered chicken runs work well too. Build them a roofed enclosure with some shelves for lounging in the sun, and maybe a dead tree for scratching.

I think it's a great idea and - I really wish more people would create cat enclosures for their cats. More space really does, mostly, translate into happier cats. I'd put a couple of boxes in the studio AND a couple in the shed. Both/and. You'll find that different cats have different preferences about different things - for example, my Orange really does prefer to make the trek 2 stories down into the basement to poop, but when he's upstairs then he will use the box up here to pee. Just doesn't like the stinkier odors in his living space! My enclosure for my cats is basically an enclosed garden that I also use (plantings, tables, chairs, pond, etc.) Designing it with your own comfort in mind you can happily spend time with your cats. Or, other people create the space just for their cats - with multiiple levels and lots of tree limbs for the cats to climb on. Have fun!

This link below has a bunch of ideas and links to pre-made, plans, and inspiration photos.

We have a patio yard with privacy fence and I added a netting material to the top to "cat proof" it. We let our cats come out with us while we are out there. They are not allowed out unattended because it doesn't keep out raccoons, stray cats or potential other harms. So for unattended access I'd build a fully enclosed secure area.

The cats love fresh and and sunshine! Just remember they may need flea control or additional vaccines depending on exposure.

Here is a link that might be useful: cat enclosures