Here is an intro to camunda dmn DMN Tutorial | Camunda
The wysiwyg modeler is easy to integrate in your phoenix application.
There is drools also https://www.drools.org/ (eclipse ide plugin for development so no webfrontent I suspect), less lightweight (but mind you: “Drools Fusion (complex event processing features)”!), I do not know if it’s easy to integrate.
About different rules languages: https://stackoverflow.com/questions/52299097/drools-with-dmn-vs-drl
