The good news is that you can create your own broker backend and plug it to the open-source Mercure Hub.
https://github.com/GromNaN/mercure-mongodb
GitHubGitHub - GromNaN/mercure-mongodb: A MongoDB Stream transport for the Mercure hub, packaged as a Caddy moduleA MongoDB Stream transport for the Mercure hub, packaged as a Caddy module - GromNaN/mercure-mongodb