Sharepoint - Is there a tool that helps with Modern UI column formatting?

I haven't used it yet but Chris Kent's ColumnFormatter client side webpart might be what you're looking for.

https://github.com/SharePoint/sp-dev-solutions/tree/master/solutions/ColumnFormatter

The one which became trending recently is SharePoint Site Designer

However, you should compare it to the latest schema from Microsoft, because MS constantly adds new features and Site Designer might have some delay in support for new elements.

UPD
It doesn't answer the question directly, but it's just another tool for convenient JSON manipulation with SharePoint Site Scripts. I misread the question :)