It's the very same problem that must be solved within mkimage:
some package lists get expanded early and some late thus having
no chance to influence apt's choices of alternatives made early
(in fact, too early).

Until that, here's another kludge...

PS: turns out that ^systemd- is not "drop ^systemd" but rather:
    systemd-analyze
    systemd-coredump
    systemd-journal-gateway
    systemd-networkd
    systemd-sysvinit
    -- thus one /really/ wants something else.