You might consider checking Jython out as an option. It's JSR-223 compliant and trivial to drop into a Java app and just expose Java objects to the Python scripts to be used as is. I've had a pretty great experience with it.

The only downside is it's stuck on supporting Python2.x so you may end up wanting to properly integrate CPython eventually. Since you're targeting running Python code that doesn't exist yet and the language differences aren't huge though, I doubt most users would mind (I wouldn't). Just an idea to consider, esp for an MVP

(One /upside/ is Jython is a Python2 interpreter fully written in Java, so the concurrency and performance may be better than CPython2 with its GIL)