My watch vibrates only once

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

If you are worried this won’t be enough, you can set a delayed start of alarm sound on the phone. And there is always the backup alarm.