Even though they are sold out, you MAY still be able to get a price drop on the solo. No harm in calling Royal, telling them you noticed price drop on insides, and ask (nicely) about getting a price drop on the solo. In the past, I’ve gotten price drops on sold out triple occupancy cabins when I saw price drops on double occupancy cabins and called them.
FWIW, I loved the solo cabin I had on Brilliance. Yes, regular insides are bigger (although a lot of that is taken up by the second bed), but I was only in it to sleep, shower and get dressed. My cabin had more than ample storage (suitcase fit under the bed). I found the location of the cabin great. While located right off the centrum on deck 4, it was nice and quiet located in a private hallway for just the 9 cabins there.