Perhaps as an alternative you may create your own API endpoint that does use the required package and host the endpoint on your own infrastructure. The endpoint could receive the new user details and POST the data to SendInBlue’s API.
To get the new user details to your endpoint you could use a Post User Registration Action indeed this particular Action is designed to do just that.
Not completely related but there’s an example here of how you might use axios to post data to a third party API within an Action.