I want to switch my mysql5.7 to Mariadb, but how should I choose the Mariadb version?

For better compatibility, you can choose the version corresponding to mysql5.7. Please refer to the following for details:

MySQL 5.5 -> MariaDB 5.5
MySQL 5.6 -> MariaDB 10.0
MySQL 5.7 -> MariaDB 10.2
MySQL 8.0 -> MariaDB 10.3