So just confirming that you only need to patch the boot image ONCE per Android version and that you can 'fastboot boot' the same patched version after each subsequent update of the same Android version.The purpose here is to have a patched image that you can boot after every update and use it to root your phone directly with Magisk. This should work at least until Android 13, regardless of how old the image is.
That being said, you can always manually patch the new boot image if you prefer to do it that way, but that will require downloading the factory zip every time there's an update. Temp booting a master image makes it easier to re-root post OTA.
Thanks!