@ Manoj’s one-line hint “A different OS version or another custom ROM Pie or LOS 16.0 ROM’s once flashed prevent a downgrade of the OS.” sounds extremely problematic.
There is a newer XEF 9-pie image from 2019-10-15 PDA G965FXXU7CSJ1 - CSC G965FOXM7CSJ1.
Have you tried to flash StockROM Android 8.0.0 with mit (Windows PC) Samsung Odin v3.13.3 ?
Here I would try to flash the four-part firmware (BL, AP, CP, CSC) of StockROM Android 8.0.0 Oero (instead of one-piece - AP only)!
Comparison also Windows PC and Samsung Odin 3.13.3.