@Elias I have read elsewhere that RDS may not need the port configured. Try commenting out the DATASOURCES_DEFAULT_PORT=3306 line and see if it makes a difference. (It may not, as the port may be set to a default 3306 in passbolt. But we can experiment if this first step does not work. )
The other issue may be security settings on the AWS end of things, like a firewall setting.