(Personally, I would suggest having one extension per driver type, as that would allow you to directly require the necessary Composer packages, instead of asking for that as an additional installation step.)
@franzliedke I like your approach with having one extension per driver type, we can definitely do this under FriendsOfFlarum to keep each individual mail driver package maintained 💯
You will need to install an additional Composer package to support Mandrill in Beta 14, as we plan to upgrade Laravel to 6.0 by then.
See https://laravel.com/docs/6.x/upgrade.
(Personally, I would suggest having one extension per driver type, as that would allow you to directly require the necessary Composer packages, instead of asking for that as an additional installation step.)