I tried multiple method to resolve the flickering issue.
Final solution from apple authorized service centre:
To Replace display.
How to find problem quickly:
1. Connect external monitor and check display.
2. If you don’t have external monitor. Take screenshot and share to cloud then check screenshot from mobile.
If screenshot or external monitor is clear view then need to replace display
If image / display is not clear then still hope to resolve the issue without display replacement
Don’t waste your time to resolve flickering issue with another method