Are you absolutely sure you flashed the correct stock firmware file for your device and region as flashing that should fix frp. Alternatively you could try flashing the stock firmware using Heimdall but I have no experience with that so you'd have to look elsewhere for help with that
Whats wifi display reboot the phone? What you meaning
Thanks for testing these, I will add them to the list.
I am NOT responsible for bricked devices, destroyed SD cards or any damage that may happen to your device.