Mostly solved with the help got here: https://discuss.kde.org/t/amarok-how-to-recover-statistics-database-lost-in-transition-2-9-3-x/32573 from Tuomas Nurmi:

According to amarok - Debian Package Tracker, only 2.9.0 was ever packaged for Debian. That one was still KDE4/Qt4 based, so it is possible that the embedded mysql/mariadb database is at ~/.kde4/share/amarok/mysqle/ and it could be copied to ~/.local/share/amarok/mysqle and everything’s there. There should have been an automatic migration from KDE4 paths, but at least I haven’t tested if it works during this decade. (Or, might also be some Debian-specific issue.)