Can you rIse the hallway ceiling height and leave the bedroom heights as mentioned? This is your best solution since the hallway is open to the open concept kitchen...

You don't see this much in photos. Usually the mouldings are painted the same colour as the ceilings which of course lowers the ceiling height even more! Vertical stripes would be fun!

Try a light warm gray

I am facing the same thing !! In the same spot Your ideas sound fab. I was going to put a door on mine as I don't have pretty books like you have though. But indent the bifold door 300 in, and make it 600 wide as the hole is 900 then have shelves on one side for some treasures I will start to collect cos I have a place now for them. Do you love the arch or are you going to square it off? Do you love the small one on the left? If not...make it a square niche and modernise them / it, as I'm not a big fan but you may love them. What you propose sounds perfect. The dark colour, nice too..or moody wallpaper on the back wall before putting up the shelves.