Your attic looks similar to mine, as does the slope. I only let my girls get to around 12"-14" tall before flowering them. This way I don't have a height problem. I do wonder about your light with the balast attached. It is quite a tall unit and you might have some fun positioning it for best results. I see some others have offered some ideas. Regardless of where you live, your attic will need some sort of temperature control. I was struggling to keep my space under 30c during summer, but now, only a few weeks later, it is getting pretty cold at night.
I built my room by using sheets of 25mm polystyrene panels, cut to fit between the framework. That has worked pretty well, even allowing me to mount an extractor fan in the polystyrene wall. I posted some shots of min, but it was before I knew how to handle posting pics, so they are just thumbnails. If you are interested, this is the link...
https://www.rollitup.org/grow-room-design-setup/185669-1st-time-attic-grow.html
I have now added a second space where the slope reduces height to around 4' and use this for vegging. I did have to back the polystyrene with mylar to prevent light leakage on the shared wall.
Best of luck and be careful where you stand!
Lefty