Comments - Migrating from Mysql 8.0 to MariaDB 10.5
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Rules customization before migration would resolve almost all the issues. Try MnMTK 2020 for MySQL to MariaDB conversion, it may help. https://www.ispirer.com/products/mariadb-migration
I would advice to use the new mysqldump option --system=user provide in MariaDB and report if it failed on the MySQL also possible tools to dump user can be found in percona-toolkit