Oof, yeah that's painful. I'm impressed you've gone to these lengths, most maintainers don't do that. They generally rely on the folks who want to use the software packaging it for their distros. Obviously there's a bit of a chicken and egg problem there, though, if you care about getting your software in front of users.
For the Debian situation specifically:
> It would've been nice if there was a quick serverless solution where you could just say a URL to look for newer versions of a single package, and both apt and dnf would remember that and update a package.
Have you considered just hosting your own Debian repo? It's basically what you're looking for here. You don't need more infrastructure, either, you can host them straight in github pages with aptly or reprepro or something, and drop new releases in there from CI.
For the Debian situation specifically:
> It would've been nice if there was a quick serverless solution where you could just say a URL to look for newer versions of a single package, and both apt and dnf would remember that and update a package.
Have you considered just hosting your own Debian repo? It's basically what you're looking for here. You don't need more infrastructure, either, you can host them straight in github pages with aptly or reprepro or something, and drop new releases in there from CI.