Integration of ETM with external tools
One answer
It would be hard to tell what to do in a simple answer, but I think (as you mentioned) you can use QM REST API calls to customize what you want.
Here is a useful resource you can visit:
I know you stated "...with external tools apart from the Adapter or Commandline adapter methods" but just in case you find it useful: https://jazz.net/library/article/809