Doing this on the Pixel 6 Pro for the first time. Unlock worked fine but when I try to flash the updated boot.img I get this error.
C:\platform-tools>fastboot flash boot --slot=all C:\platform-tools\magisk_patched-24314_zbwXC.img
Warning: skip copying boot_a image avb footer (boot_a partition size: 67108864, boot_a image size: 100663296).
Sending 'boot_a' (98304 KB) OKAY [ 2.315s]
Writing 'boot_a' (bootloader) unable to update boot image: use unmerged...
FAILED (remote: 'failed to flash partition (boot_a)')
fastboot: error: Command failed
See the last few posts above.
Magisk must be done in a very specific manner, and IMHO is the hardest part of the entire process.
Make sure you're using the boot.img within the second zip folder after unzipping the first factory zip folder.
Then you must make sure you have the proper version of magisk, install it first, then follow the patch boot.img steps.
While I cannot promise you what went wrong with your magisk install, I CAN promise you my step by step in the OP works 100%, as do the guides also mentioned in the OP.
Start fresh right after unlock. Read every step, understand it, then perform it. I know the impulse to just go balls to the wall and get a device rooted, but manufacturers, even Google to a degree, have made root harder to achieve, requiring more difficult methods to install.
THANK GOD FOR DEVS!!!
Make sure you have the following;
Latest SDK Tools
Proper version of magisk
Correct factory image
Correct boot.img file
All this is mentioned, with links, in the OP


Best of luck!