As far as I know, MariaDB isn’t supported at all (maybe it was years ago, but in the last few years it wasn’t). I’d recommend to migrate to MySQL first, before migrate to PostgreSQL (e.g. because the migration script supports MySQL only).
For MariaDB → MySQL: https://blogs.oracle.com/mysql/post/how-to-migrate-from-mariadb-to-mysql-80
For MySQL → PostgreSQL: Automated PostgreSQL migration - Mattermost documentation
For now, I don’t have much real-life experience with these types of migration, though.
What are the points in the docs that are too complex?