You don’t need to mess with system folders. Plasma can load a wallpaper from any location in the filesystem where you have the permission to read files.
However, that said, there appear to be various ongoing reliability issues in Plasma for lots of people — albeit strangely enough not for everyone — ever since Plasma 5.25 was released, and at this point in time, many of those issues continue to endure in 5.26. I would therefore advise you to read and/or participate in the thread below. 