That was fast... I've finished already. I've added the waistband - it wasn't easy. Straight lines are not my thing and I did screw it up badly on the first try. That led me to 30 minutes of doin nothing but cutting off the seam and restarting. I took it slow, stopping the sewing machine every few seconds to check what I was doing was correct.
After that, I added button holes for the smaller buttons that fasten the waistband shut.
I made the skirt with a fitted waistband so I can't adjust it and, funnily enough, I've lost weight since I started making it. It's a bit loose now but I can put a belt on it. There are some changes that I would make - the main one being that I'd make the waistband wider. I'd probably make the pleats bigger too. It looks really nice and I have quite a few tops that this can go with. If it survives the washing machine, it will get a place in my wardrobe.