You’ll see all kinds of different numbers thrown out for required and preferred dimensions. A very common recommendation for breeds your size is to put the first roost 12” off the wall and use 12” separation for the next. I have some like that in one of my coops and they use them, but it does look crowded. Keep your current layout but move the door down toward the nest end. You can fit two roosts and a droppings board in.
You’ll often see people warn about heavier chickens hurting their legs by jumping down from the roosts. I find that if mine can spread their wings and fly down they don’t land that hard. If that space is too tight or filled with feeders, waterers, or other stuff, I can see that being more of a problem. I’m not sure where you are getting your 35” in the middle but if it is clear, that’s probably enough. If is full of stuff, nope.
You’ll often see people giving all kinds of magic numbers for how much space you need per chicken. I don’t believe any of them. What’s important is usable space, not just for the chickens but for you. I think you are making a walk-in coop so I’d make it big enough for me to walk in and do things. That will make it plenty big enough for six chickens.
I don’t know your stud spacing but the basic difference for the walls between a 4x8 and 6x8 is you buy one extra sheet of plywood or whatever 4’ x 8’ paneling you are using and split it down the middle. You may need one extra stud on each end.
Consider how you are doing the floor. If it is dirt you just need to adjust your foundation. If you are flooring it you will probably need more material.
I’d make one narrow end of that eight feet tall instead of six. 2x4’s and paneling come standard in 8’ lengths. Make the other end at least six feet. You and others need to be able to stand up in there without banging your head. That hurts, I know. That gives you a good slope so rainwater runs off instead of pooling on a flat roof.
I’d use 8’ 2x4’s across the top of that to give a foot overhang on each side. That way you can leave the top of the side walls open (covered in hardware cloth for predators) for good ventilation up high and keep the rain out. Slope it so the runoff flows away from your run. Put your door where the runoff from the roof is not hitting you when you open the door.
Yes, your costs will go up some, but you will have a much more usable coop. Most of that is for your benefit, not the chickens, but the chickens will not complain about having extra room.