That’s quite clever. If you place the paintings over glass then it will still appear as a “window” when you zoom out. I was going to suggest windows as well. But your painting idea is really good.
To jazz it up a bit you might add some columns or stairs. I often place wooden stairs on the ceiling as “rafters”. if you can find any red marble or lapis lazuli you might drape a few blocks of that color backdrop as a “tapestry”.
I’ll also share a wee little crenelation trick I learned. If you sink a door down into the wall walkway so that half is in the floor and half is above, every other block, it will give a nicer crenelation effect and your BH will still walk through/behind them.