My roosts are about 4 feet high. Your Brahmas are probably bigger than anything else I have, but mine are similar to your others. Mine can fly straight to the roosts. I suspect yours could too at five feet. But my nest boxes are along a side wall two or three feet back from the roosts. Almost all of mine use the top of the nest box as an intermediate step. I think you would be better off with an intermediate step for them to use on the way up. Another roost half way up would not be a bad idea.
I'm not sure why you settled on five feet. Mine are about the level of the bottom of the windows. The prime preferred roosting spot is next to the window where they can see out. As far as ventilation goes, they don't need to be above the windows. Equal to or below works well.
I suggest you consider puttting the roosts as low as possible and still keep them noticably higher than the nest boxes. They have to be noticably higher than anything else in there so they will use them. I think you know that. But there are a couple of reasons I suggest trying to have them as low as reasonable. One, chickens are fairly clumsy flyers. Mine fly straight to the ground instead of using an intermediate step. The higher the roosts are, the more horizontal room they need to fly down without hitting walls, nest boxes, feeders, or anything else. In an 8'x8' coop, that is probably not a big problem for you.
The other reason is that the larger birds, like Brahmas, can injure their legs when landing from the roost. It is not that they break legs or jam joints each and every time they come down. They don't. They spread their wings and fly down so they are not just launching themselves and cannonballing down. If five feet is a reasonable height for your roosts considering the layout and such, they will probably be OK. But there is always a chance they will hurt themselves. It is not something I would obsess about, but I think it is good practice to put the roosts as low as reasonable to reduce the chances of an accident.
Good luck!
Editted to add:
The normal recommendation is about one foot horizontal separation on the different roosts. Mine are all the same height but looking at them, I'd probably go a bit more than 12" if they were staggered height. 12" just looks too close for falling poop.