About GSI maker you are right
The order you install magisk doesn't matter in this case because magisk uses the boot partition and not system. You can install it normally by recovery after vbmeta. About the magisk menu, you will only be able to use it after I manage to decrypt the internal storage
hi :
I put the doc here :https://xdaforums.com/t/rooting-custom-rom-your-brand-new-redmi-note-9-t-for-the-noob.4296447/
please comment