TL;DR
Hope I’m not suggesting anything unrelated or obvious…
But changing the wallpaper should work like this:
dconf write /com/deepin/wrap/gnome/desktop/background/picture-uri "'$pic'"
where $pic is a variable containing the full path to a picture file.
Does it still work that way now that most of the gnome stuff has been removed from deepin and it is based on kwin?
I am sorry you were disturbed. Please don’t waste your time on my posts. Deepin always gets me in trouble 
I am switching to another DE.
Why?, snaps until I understand you but flatpaks?
At least the flatpaks are here to help me and I like that
I just like the AUR better that’s all. If i have a choice i prefer flatpaks over snaps. But I prefer the AUR on Arch.
Same for me, except I outright refuse to use snaps. 
When I want to install a program, this is the order in which I look for it:
Official repos > AUR > Building manually from source code > AppImage > Flatpak
I have installed, uninstalled, reinstalled (multiple times) and spent a long time setting up over the past few days. I finally have it fully stable in a VM. The only issues I have remaining are the “failure to start kernel module” errors on startup (that don’t seem to impact anything except causing startup to take a while) and a bunch of missing app icons in the app drawer from apps installed after the original install. I did need to install one app (kdenlive) as a snap to get it working when the one in the repo would look like it was going to open and then do nothing. For the foreseeable future, I will keep it as my regular use VM. It might eventually become a DD for me if the Elementary OS devs keep dragging a$$ releasing 6.0 since 5.1 is my current DD.
As long as you don’t mind the fact that something new will be broken every time it updates you should be in good shape. 