Maestro,
None of us are getting any younger..........it took me 35 yrs to realise the dream of being able to record me and my guitar

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OK, I had typed out a lengthy reply and lost it all, so this will be briefer. I have done an overdraw of your design, based on...... 1) it is generally accepted that a control room is better being longer front to back than it's width and the need for symmetry, 2) the need to increase booth sizes. This is only a very basic suggestion and would need considerable refining but it may give you some further food for thought.
The red lines represent walls, the blue are windows and the black are doors. An alternative for access to the vocal booth would be to replace the window from the control room with a door (or pair of), either discarding the doors between the booths or leaving them in place.
A suggested plan of attack..........get your floor plan sorted out, this way you should know of any access points for electricity, plumbing, etc., at the same time give serious consideration to sheeting the inside of the existing structure, walls and roof area using rockwool between the wall studs and sealing any possible cracks that could be a weak point for sound to escape. Plan on building the control room and booths inside AND NOT attached to the existing walls (small boxes inside a big box). If the work to the existing walls is satisfactory, it is possible that you may not have to sheet the outside of the new wall frames, just pack the void with rockwool prior to sheeting the inside, obviously, sheeting both sides will give even better sound isolation.
When working with an existing and limiting structure, there are always going to be compromises between what we would like to do and what it is possible to do. Ultimately, the first priority is always attempting to maintain the integrity of the acoustics.
As I said before, I would seriously suggest posting over at John Sayer forum, as you will get excellent advice from people who have been there and done (or are still doing) that.......it may take time to get a reply but it is usually worth the wait.
