Thank you for using Apple Support Communities. It's my understanding your iPhone 8 only vibrates once when on silent. You've done a great job trying to get this resolved so far. I'm happy to help.
You can set and create vibration patterns in Settings. Please review the steps below:
iPhone User Guide - Sounds and hapticsSet sound options. Go to Settings > Sounds & Haptics (on supported models) or Settings > Sounds (other models) for options such as ringtones and alert tones, vibration settings and patterns, and ringer and alert volumes.
Set vibration patterns. Go to Settings > Sounds & Haptics (on supported models) or Settings > Sounds (other models), then choose an item from the Sounds and Vibration Patterns list. To select a pattern or create your own, tap Vibration.
Please let us know if the issue persists.
Have a wonderful day!