I would like to be able to hover over a message in Slack + from the "More Actions" menu, use the shortcut "Create a task in Sunsama" to create a new task (so I remember to follow up on random things people have asked me about through the day). Let me know if this is in scope of this ticket, or if I should create a new one. Thanks!

Is this a workaround for a full-fledged Things app integration?

In Things, a ToDo has a Share menu...

There's also a way to copy "link" to the task, giving you the unique ID of the task (ex. things:///show?id=KGgSvsA5arBYdnW3xpcNDm). This would be useful in the Sunsama task description to backlink for the info/context managed in Things.

But you could use the URL scheme to update info/mark completed also in Things - allows update access if you set up the auth key for URL - https://culturedcode.com/things/support/articles/2803573/

There's also Apple Scripts:

And their new Apple Shortcuts Support:

You're right, this won't work on Android. But if you're using Things, you're not using Android.