I really wish Mermaid and PlantUML had compatible syntax, or at least more overlap for the basic structural stuff. On a day to day basis I'm writing either PlantUML in Confluence (via a plugin) or Mermaid in GitHub, and I almost always have to open up the reference or review an existing diagram to refresh myself on the different syntax.

FYI there’s also a Mermaid plugin for Confluence, we use it extensively. If you don’t mind losing some features from PlantUML for the sake of consistency.