Do not ring bark - see https://www.rhs.org.uk/advice/profile?PID=922. Its bad practice and not recommended.
I have a 200ft long native hedge which I have inherited and have been managing this for 3 years. Its a complete mixed bag from extremely tall and leggy to very wide and in some places gaps in it.
Unless a plant is obviously dead, (you need to replace this) then you need to start by stopping the top growth on a regular basis so cutting this twice a year and not letting any leaders grow. If its too high then lop off. Mine was 2.5 meters in places so i have taken off 1 meter (well a tractor man did). It may not look its best next spring but I am trying to get the whole hedge in relative uniformed height and thickness. I think I have another two years to go before I achieve this. Its long term but once you get the shape and height right the you will be able to manage it and it will look superb. I will try and post some pics of what it was like this summer and what its like after its haircut.
In relation to the deadwood. If you start cutting on a regular basis you will create the deadwood. I also add to this by chucking leaves from the trees into the hedge too.
Hope that helps