use the module INPUT variables within advanced section to configure other options. e.g. username input variable can be used to configure the username for master DB user.
Additional parameters can be configured with in the rds-mysql block of advanced section e.g.
{
"advanced": {
"rds": {
"rds-mysql": {
}
}
}
}
Considerations
Mysql DB instance with aws rds flavor can be configured with just Writer only. Reader is not mandatory