I've amended the documentation with missing pieces, that were required to build a robust solution.
A few weeks ago I wrote an article detailing the exact process and what arguments are needed to make it work. I believe it could be useful for people looking for usages of the CLI.
You can find the article on this url: https://advancedweb.hu/how-to-integrate-plantuml-into-other-software/ . If you find it useful I'd be thrilled if you'd put a link to it in the docs.
I know I can edit the wiki myself, but that might be considered spamming and that's why I'm writing in this forum. Feel free to close this question if not appropriate.