first things first: ...you have to unlock your bootloader for get this working ...you will need Magisk and default ADB and fastboot tools ...I won´t get into the bootloader unlocking part or essentials
...I suggest getting "Oxygen Updater" at Google PlayStore and download your FULL rom ...maybe it will complain about the "wrong" version (India/Europe) but I know I´m in Europe and it´s a CPH2409 ...so I got the most recent full update, in my case: CPH2409_11_A.11 ...you have to download the same version as actually installed
...after downloading the File, get the PayLoadDumperGo
from XDA Member ssssut
and extract the downloaded file ...maybe you will need to add "liblzma-5.dll" to the directory ...I think you will manage it by yourself ...you will only need the "boot.img" and "vbmeta.img"
...copy the boot.img to your phone and patch it with the usual Magisk method (install / choose file) and copy back the resulting magisk_boot.img to your pc.
...now the tricky part ...and yes, I got the "broken Boot/Recovery Image Screen" and yes I was scared for just a few seconds ...but I solved it and here is how:
reboot your Device to fastboot mode (unlocked) and type:
fastboot flash boot .\magisk_boot.img
DO NOT REBOOT NOW!!!
...you will have to flash vbmeta.img like this:
fastboot flash --disable-verity --disable-verification vbmeta .\vbmeta.img
...now it´s time to reboot and when it´s done just start up Magisk Manager and finish the setup and you´re done ...simple as can be