Thank you tejing! The PartOf should be OK. It makes that stopping the service also stops this timer.
Oohh, I got it!
The enable option for timers and services works differently than I expected:
Setting enable to true (the default) does not enable the unit in a systemd sense or in the sense of other services.*.enable. It only stands for not masked and masked systemd unit.
For reference, with enable = false I got links to /dev/null:
$ ls -l /etc/systemd/user/org-taskjuggler-make-reports.timer
lrwxrwxrwx 4 root root 127 1. Jan 1970 /etc/systemd/user/org-taskjuggler-make-reports.timer -> /nix/store/a0y3ph7d5a5zms6j73dw2wpc4fxcg7l3-unit-org-taskjuggler-make-reports.timer-disabled/org-taskjuggler-make-reports.timer
$ ls -l /nix/store/a0y3ph7d5a5zms6j73dw2wpc4fxcg7l3-unit-org-taskjuggler-make-reports.timer-disabled/org-taskjuggler-make-reports.timer
lrwxrwxrwx 156 root root 9 1. Jan 1970 /nix/store/a0y3ph7d5a5zms6j73dw2wpc4fxcg7l3-unit-org-taskjuggler-make-reports.timer-disabled/org-taskjuggler-make-reports.timer -> /dev/null