One similar self-hosted solution is https://gotify.net/ , which also has Android and web apps and a similarly simple way to push messages using curl/http.
The only way I've found to do encrypted notifications is using Signal and signald, through a library I wrote:
https://gitlab.com/stavros/pysignald
signald isn't as reliable as I'd like, but it works well enough. Still, if there were a simpler alternative, I'd switch.