Rabbit Grow Out Pen

I got into meat rabbits recently. I have two does, a Rex and a New Zealand, and one buck, a Californian. The does are pretty wild, but I would feel really bad if they always lived in the popular wire cages (although I definitely understand why they're used for large operations). So, I made them each their own two story cages which they looove. They've never tried to dig or chew their way out and they get a fair amount of exercise. The cages get completely cleaned out every weekend and that's worked really well for me so far. The buck is my pet who stays in the house in his own little set-up.

Both of the does are in the process of weaning their current litter, so I'm looking into how I want to grow out the kits. There's nine of them, but I suspect there will be more next time they kindle, since this was the Rex's first time. I'm estimating that I'll have 10-15 rabbits to grow out at a time. I really don't want to put all the rabbits into their own separate little cages, so I'm thinking about making them a little shed. We have a bunch of leftover concrete mix, siding, and other supplies from building our barn so money won't be an issue.

This shed would be something like 8 x 10 and have a concrete floor. I'd use the metal siding for the walls and untreated wood, sturdy pvc piping, or T posts to add support on the inside. A human door on the south side and two or three 'window' like holes with welded wire to keep hawks/etc. out, to add air flow. It would be close enough to our barn to run extension cords for heat lamps/fans in extreme weather. I live in Nebraska, so our year round temps usually range from around -15F to 105F.

My parents have given me the ok for this whole project since raising rabbits as livestock is so much simpler than cows or sheep. Buuut, I want to know all the flaws about this idea, so please be critical. What do I need to do differently? Do they need more space? Would the cement floor be okay (with bedding changed every week)? I plan to use an automatic waterer and feed them hay, alfalfa, and pellets.

Tons of poop and pee = sickness.
Wire floor will keep them clean.

But if you don't want them on wire, just keep the shed floor clean, clean, clean.
Food and water off the floor and where they can't get their feet into it. Actually, having the food and water over a wire floor would be best. So any fallen food won't get contaminated and then eaten by the rabbits and any leaking water will be contained and hidden away from them, again, to keep them clean.

DO NOT use heaters, extension cords, etc. Unless you want a fire and everything gone, do not do it!
They have fur, they need nothing else!
Use straw for bedding and they'll be fine. Thicker in winter.

Toss litters together the same day. Even a couple weeks age difference is no issue.

Make sure to have several food and water stations. They will force out the weak and timid and you'll have slow growers or ones that just die or get sick easy.