Hi @tksol95 you are correct that at this time, a different SMS provider would need to be setup as a custom action. We have additional providers on the horizon, so please reach out to me directly to let me know which provider you would need to have as a connector.
Regarding the Custom Action - this need only be setup once at the configuration level. You can determine which portions of the custom action payload are static and which values are dynamic. Once you drag the action onto the journey canvas you can customize the payload with dynamic values that include values from profile and contextual attributes.
Beyond SMS, we have many customers that are using this workflow to send MMS messages as well.