Hey /diy/

So there's a room with a tilted/slanted celing such as pic related.

I wanted to strip the existing dry wall from it, then make the celing level and redo the dry wall so it's a flat and I have some roof space and it looks a bit better.
It will give me space to add insulation as well as run some cables.
I can do all the drywalling myself but I don't know how to construct it in a way that deletes the black line and replaces it with the blue line without looking like shit or sagging.


Any pointers for how to go about/find the correct information?
It would be a timber frame roof.

Thanks /diy/