Good evening, is there a cause/reason the advancedsettings.xml will not work?
Tried with a raspex version/image (kodi 18.3 Git:20190718-b0e4133d0f) and my mariadb is able to connect.
For the w0man acceptance factor i would prefer the libreelc image instead
advancedsettings.xml is working fine (41,000 users can't all be wrong!) and if it's not working for your then you've probably made a mistake somewhere, or something else isn't working as expected, in which case please post your kodi.log as this typically reveals the underlying issue.
If you're not able to connect to a MariaDB server from LibreELEC then enable "Wait for network before starting Kodi" in Settings > LibreELEC > Network > Advanced Network Settings - this is a common issue as Kodi frequently starts before the network has been brought up.