Installing MariaDB wasn't a problem, but I couldn't get Adminer work (neither with Apache2 nor with Lighttpd).
At last I had a working combination of MariaDB, Apache2 and phpMyAdmin.
Tests with the storage of all 50,000+ historical records of my KlimaLogg Pro showed storage times of 9 hours for a MariaDB and 2 hours for a SqLite database.
This made me switch to Sqlite databases.
On my RPI 2B I now run Weewx 3.2.1, ws28xx and klimalogg drivers, Sqlite3, lighttpd and phpLiteAdmin
On my Synology DS209+II I run weewx 3.2.1, ws28xx and klimalogg drivers, Sqlite3, Apache2 (part of the Synology web services) and phpLiteAdmin