Thanks for your walkthrough. I managed to dump the ROM using Wwr + SP Flash tool. I went ahead to patch the boot.img in Magisk. However when I try to flash the patched image in adb I get the Error "Cannot load patched_boot.img". The device is well detected in both adb and fastboot and I saved the patched file in the same directory as adb tools on C\: from where I'm running the commands.
I'm also drawing blanks when I try to flash the image using the SP Flash tool. I am getting BROM ERROR : S_SECURITY_SECURE_USB_DL_IMAGE_SIGN_HEADER_NOT_FOUND (6045).
Then tried to use the Write Memory utility (as advised by someone for whom that worked) and get Error 1002: S_INVALID_ARGUMENTS.