Single Pipeline Slack Notification

I currently have a pipeline setup like the following

Originally when I created this, I had all workflows within the pipeline publishing their success/failure to our slack, which got a bit cluttered. I then tried to make it so that the final workflow (NightlyPublishTest) would be the one in charge of publishing the slack notification so that we only get a single notification for the whole pipeline.

The issue as you can see in the example above, is that whilst some of the test workflows failed, the NightlyPublishTest doesn’t have any idea of this, and just sends a successful notification even though it should be marked as failed. Note that I have the stage for NightlyPublishTest set as always run despite failure, as it will be skipped otherwise.

Any suggestions on how I could get this working?