I'd go lower on the roosts, ~8" above board....
...and fewer supports(maybe one, if any) underneath roost,
they just get in the way of cleaning and are probably unnecessary.
Go with none and can stick something under if too much sag occurs.
My thoughts here:
https://www.backyardchickens.com/articles/coop-stack-up-how-high-stuff-works-well.73427/
Those will likely be fine (is the actual width measurement 2.75?), roost size is a much and hotly debated topic here.
Use screws to build everything, deck and drywall screws are easy to...
Cool!
Will you also have separate areas for storage of feed and supplies?
And one or two other areas for brooding chicks and/or isolation hospital pens?
Heat is fine, as long as it's safe, but don't restrict ventilation to 'hold heat'.
Plan to keep traps set in the barn but outside the coop.
Keep your feed in a metal container... garbage can, old freezer, etc.
Spill proof feeder helps too.
Works here.
Only the very smallest mice....the bigger ones get stuck.