Right, finally figured this out.
The AI's answer (now hidden) pointed me to a custom driver, which set me on a very difficult path to no success. Which is probably due to my lack of knowledge, rather than the answer...
In the end, I found this thread which provided me with the solution.
Long story short: just pass the config into the mailable itself so it gets stored in the job. Set the Mail::SymfonyTransport inside the Mailable's build function, which is run during job-execution.
//Update Later on I realised that the Mailable's build method is called at time of sending. So instead of having to pass the config in the mailable, I now pull it live in the build method.
// Sending the notification malil to the queue
Mail::to($model->account->getSetting('adminEmailAddress'))->queue(new NewCommentNotificationToAdmin($model));
// The build method in the mailable getting the config before sending
public function build()
{
// Build a custom transport based on the sender's account config
Mail::setSymfonyTransport(Mail::createSymfonyTransport($this->comment->account->getMailConfig()));
return $this->markdown('emails.comments.notificationToAdmin')->subject('New comment on '.$this->post->title);
}