For the POC there will be very little UI design involved, if any, except for possibly displaying the workflow configurations in the UI as read-only.
As a next step, it would be helpful for you to research other workflow builders and how they are done. Here's examples:
Slack workflow builder: https://www.youtube.com/watch?v=2zRBYwrkUfY
xMatters Flow Designer: https://www.xmatters.com/features/flow-designer/
Microsoft Flow: https://flow.microsoft.com/en-us/
kore.ai bot builder (I can get you creds if needed) https://bots.kore.ai/botbuilder/login
The outcomes would be:
pros and cons for each system (focused on UX/UI)
exploration of how we would do workflows once exposed to the UI
Issue created from a message in Mattermost.
No testing required.