Interesting project. We build and maintain a significant cross-platform JavaFX app, and use a combination of JLink (to create a smaller executable with bundled JRE), install4j (for the native .app/.exe wrapper) and Packages (macOS)/InnoSetup (Windows) for the installer. The installer has a non-trivial amount of scripting under the hood so being able to customize that is really important, at least for us.

I wonder if you won't run into trouble because of the Mac codesigning/notarization though. I think this poses a significant legal risk to you, as you are now distributing potentially malicious software signed with your certificate. Also I'm pretty certain Apple doesn't like this "code signing as a service" approach.

But hats off, cool project!