You are digressing somewhat from the topic of this post: running mir-kiosk within your desktop environment.
What you are trying is closer to Install mir-kiosk and chromium-mir-kiosk under Ubuntu 18.04 Server (I know you’re not using chromium-mir-kiosk or Ubuntu Server, but this is about setup on a classic system.)
When you say “at here I can see a black fullscreen” are you seeing the Orange “splash” as mir-kiosk starts and the cursor? Or just a black screen? What do you see in sudo snap logs -n 50 mir-kiosk?