With recent kernels, watchdog is not a simple misc device anymore but there can exist multiple instances. This means that a system can have /dev/watchdog0 (--> e.g. CPU internal watchdog timer) and /dev/watchdog1 (external watchdog circuit) where watchdog1 is more reliable (e.g. because it generates reset signals for external devices too).
/dev/watchdog still exists in this setup but equals to /dev/watchdog0 (misc device is created for the first wd only).
Hence, please allow to configure the used watchdog device in a first step. In a second step, multiple watchdogs with corresponding 'Wants = dev-watchdogX.device' entries should be supported to cope with changed device names due to random/parallel module loading.
The first part implemented as https://github.com/systemd/systemd/commit/8a2c1fbfc3. If support for multiple watchdog devices is still needed, please open an issue on the new bugtracker on https://github.com/systemd/systemd/issues/.