Removing a timer created with “systemd-run –on-calendar”

You can solve the duplicate by creating two directories: foo-at-shutdown.service.d and 'foo-periodic.service.d`.

In one folder create a file named foo-shared.conf.
Use a symlink to add it to the second folder.

These are "drop-in" units, which will get pulled into the main config files. They are documented more in man systemd.unit if you search for "drop-in" there.

I think having the two service files, a timer file and the shared drop-in unit files is a reasonable approach here.

For the simple use case, use WantedBy=timers.target. See man systemd.special:

timers.target

A special target unit that sets up all timer units (see systemd.timer(5) for details) that shall be active after boot.

It is recommended that timer units installed by applications get pulled in via Wants= dependencies from this unit. This is best configured via WantedBy=timers.target in the timer unit's "[Install]" section.

Timers get a dependency of Before=timers.target by default. And, if you check man bootup, you'll see that basic.target pulls in timers.target as a dependency. So I think WantedBy=basic.target would appear to work OK for most cases (same for default.target, which is usually multi-user.target or graphical.target, both of which come after basic.target). But:

timers.target is pulled-in by basic.target asynchronously. This allows timers units to depend on services which become only available later in boot.

So a more complicated timer that depends on some other service unit would be better off depending on timers.target rather than any of the others.