You can still use sysvinit, I've already tested servers and desktop builds.

From my build box:

  chroot $MOUNTPOINT/ /bin/bash -c "http_proxy=$aptproxy apt-get -y --purge --allow remove-essential install sysvinit-core sysvinit-utils systemd-sysv- systemd-"

There is a weird depends you cannot get around without simultaneously removing and installing in parallel. A Debian bug highlighted the above, with a "-" for systemd-sysv- systemd- as a fix, along with allow remove essential.

After this fix, sysvinit builds with debootstrap were almost identical as to bookworm. This includes for desktops.

As per with bookworm through buster, you'll still need something like this too:

  $ cat /etc/apt/preferences.d/systemd

  # this is the only systemd package that is required, so we up its priority first...
  Package: libsystemd0
  Pin: release trixie
  Pin-Priority: 700

  # exclude the rest
  Package: systemd
  Pin: release *
  Pin-Priority: -1

  Package: *systemd*
  Pin: release *
  Pin-Priority: -1

  Package: systemd:i386
  Pin: release *
  Pin-Priority: -1

  Package: systemd:amd64
  Pin: release *
  Pin-Priority: -1