Actually, it ran stable when I set it up using your guide. The reboots, which occurs very random, first appear after refreshing the mir-kiosk snap to --edge from --stable. What I wanted to achieve was to change the startup URL, hide the cursor and prevent reload of the URL as I looking into using this for a digital signage solution.
But the hidecursor config in chromium-mir-kiosk had no effect and @alan_g instructed med to use mir-kiosk from --edge which has a configuration file where I can hide the cursor successfully.
With those settings I get the occasionally reboots. At first I thought that it may have to do with the snap and system updates but it is not in time with the timers that handle the update checks.
This is my config for chromium-mir-kiosk:
snap get chromium-mir-kiosk
And this is my config file for mir-kiosk:
Feel free to try the URL and settings out. It is a simple HTML Menu.