There is one stupid thing about CentOS 7 that made this rather tricky: CentOS 7 by default comes with MariaDB 5.5, while Debian 9 has 10.1 at the time of writing. Removing MariaDB 5.5 and adding the MariaDB repositories to yum helped me migrate to MariaDB 10.3.
After installing the new version of MariaDB I rsync’ed /etc/mysql/ and /var/lib/mysql/ from the old server to the new one (and deleted ib_logfile0 and ib_logfile1) and started MariaDB without any issues.