apt install openrc && apt purge systemd
Rebooted and it worked perfectly. Now, I get that Debian doesn't really support OpenRC[0] (or sysvinit), and it could break in horrible ways when bullseye goes stable, or get removed entirely, but... I don't see why we need a fork before that happens? It seems like it's a lot of work to maintain a distro fork, when I feel like that effort could be more productively redirected to stronger maintenance and advocacy of OpenRC and/or sysvinit in Debian itself?
[0] Debian's openrc package hasn't been updated in a little over a year, which is indeed concerning. sysvinit does seem to be more actively maintained, though.
The problem isn't systemd's init. It's probably fine.
The problem is the systemd project is taking over the userspace with mutually cross-linked modules.
Devuan was started by a group of Debian maintainers who disagreed with the direction systemd was taking Debian.