[schapplm]

Right now I'm using a MariaDB-container as my Kodi database on a unRAID-server, do I point this to the same database or do I set up a new database?
Can you install this script in Headless-kodi? That way I should'nt need to install it on any of the clients, correct?

You should set up a new database (e.g. "WatchedList"), because the Kodi database ("MyVideosxx") gets renamed with every new version.
I am not sure, what you mean with "Headless-kodi". From my understanding, the addon only works on the clients in the Kodi GUI (where you navigate through your media database). If you use a central MariaDB database for all clients, then in principle you only need one client to have the WatchedList addon installed.