My watch vibrates only once

On the Wear OS platform and ONE UI Watch (Galaxy Watch 4 + 5), we use gentle vibrations for an even more natural wake up.
It starts with a single vibration, another in 30s, then again in 20s and so on. After a few minutes, the vibrations become continuous.

If you’re worried that’s not enough, you can set a delayed start of alarm sound on the phone. And there is always the backup alarm.