FYI, automating notarization with TravisCI should be possible:
https://blog.zeplin.io/dev-journal-automate-notarizing-macos-apps-94b0b144ba9d
I don't currently develop for macOS, but I'd be glad to help out with testing on a Catalina installation.
Thank you for all your work on Joplin @laurent @tessus et al!