All the help that members have given you, and not one thank you........ :rolleyes:
My 'normal' method when taking an OTA with Magisk installed (which is the method they outline on their help pages:
Check to see if OTA update is available (don't start DLing yet)
Open Magisk Manager, select uninstall at the bottom, then select...
Darn, everyone is blowing up their Z3s. I can share the TWRP with xbrain as well.
OTOH here is the link to my TWRP https://www.dropbox.com/sh/pa6bfxrjb8vjr8j/AABl37l2-yx-JzMAlNMg9OBna?dl=0
OP follow this exactly, and you will have no problems. https://forum.xda-developers.com/showpost.php?p=80039116&postcount=778
Uninstall Root, take the OTA, re root.