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

If you have the WireGuard config from Mullvad already, just edit your wg.conf files on client devices to route all traffic via the Mullvad servers. Basically replace all the values of the [Peer] block with Mullvad values.

If you can share your Mullvad wg config file and your wireguard-server config file here, we can sort this out together

Edit: actually since your only goal is to increase the Mullvad device limits, why not just use Mullvad-provided confs directly in your client WireGuard apps? Should be straightforward to do