Mailsystem + symfony mailer?

I haven't used Drupal Symfony Mailer → contrib module, but I understand it's popularity has led it to eventually be included in Core. If I understand correctly, the Drupal Symfony Mailer contrib module uses the sendmail transport, but the module allows you to configure a different transport such as SMTP and therefore you could use Brevo's SMTP rely. The Brevo module would be very similar to the SendGrid Integration → module in a lot of ways in that it uses SendGrid's HTTP API and the Drupal Mailsystem module to switch senders.

Yes correct, so it might make sense to provide Symfony Mailer integration here, adding Brevo as new provider ("3rd party delivery integration").

Looking at https://www.drupal.org/project/symfony_mailer → and the documentation pages I can't yet find docs on how to provice that, but we could as @AdamPS, he's really doing a great job there and will surely help to better document this.

From my perspective, with the end of Swiftmailer, symfony_mailer will gain popularity and Mailsystem (which is no more recommended with symfony_mailer) will be less relevant in the future.

I loved Swiftmailer and Mailsystem, but I think it makes sense to focus on one full-featured solution here.