Ikea has some that i use often they come in different sizes.

I think the shelves would be better as there's not this huge room taking up space. What did you end up doing?

We’ve renovated and are just waiting on the guy to come and fit our laundry cabinets..... he said he was coming one day.... then didn’t show up, gave another day and time ...didn’t show up...and then said last Thursday 8am... well it’s now 1.45am Mon morning and he hasn’t had the decency or courtesy to even ring or return our calls .... so we are now in the same position as you... I see hes here on houzz so, maybe a review will warn others ... anyway good luck with your laundry!!

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.