It finally worked! I did it by doing all the steps using HxD editor and not doing any of them using Linux or dd commands.
I selected then removed the first 0x200 bytes, inserted 0x30000 bytes at the beginning containing zeros, and selected then deleted 0x30000 bytes of FFs at the end of the file and flashed it using TFTP.
Thanks for your advice on using this application, I hope we can install a stable version of OpenWRT on our devices soon!
I have uploaded the edited stock file for the above version (Build 190125) for future reference: