Hello! Thanks for the bug report. Have you tried if your scenario works with the latest MariaDB 10.5 in unstable? Currently all development efforts go into 10.5.
- Bug#974910: mariadb-server-10.3: Multi-instance start-up... Chris Linn
- Bug#974910: [debian-mysql] Bug#974910: mariadb-serv... Otto Kekäläinen
