I wonder if you've come across Monarch? https://www.datawatch.com/in-action/monarch-desktop/ https://www.datawatch.com/in-action/monarch-desktop/ It's a well-

I wonder if you've come across Monarch? https://www.datawatch.com/in-action/monarch-desktop/ https://www.datawatch.com/in-action/monarch-desktop/ It's a well-known commercial package that's been around for ages and when I saw it they seem to have some good ideas around UI for data prep (e.g. like seeing your dataframe at every stage, reordering operations and flipping back and forth) that could further inspire your GUI.