So what are the alternatives that work with both Alpine and Debian?

My opinion is that you’d be better off sticking to the OOTB init system that these distros provide, and hop to an entirely different distro that supports the init system you want/need. And for distros that do support multiple, stick to their guides carefully. Would save from a lot of incompat quirks and unsupported bugs, since these things can be as integrated into the main system as the package manager, if not more.

Conceptually though systemd can be a bunch. The s6 dev put out these definitions for the way he conceptually breaks down the entire init/service ecology into small pieces so have a look (ou may be interested in his full post explaining the motivation behind s6/s6-manager too). And since you’re on Alpine, see their plans for a future init system.

With that said you may wanna try out dinit on Chimera Linux. They’re one of the unique distros that offers this and some other cool things