I am no specialist for Magisk as I don't use it myself. However, if you have to restore the boot.IMG after each ROM upgrade, you will never get the kernel patches coming with the ROM updates.Do you have a TWRP backup from the previous release with root?
Then you have to restore only the boot partition with TWRP (without data and system!).
You have to do this after every ROM update.
If you have success with this method you should make a TWRP backup only from root after every Magisk (release and canaries) update.
Isn't Magisk supposed to put some kind of "survival script" into /system/addon.d to re-patch the boot partition after a ROM update?