I then started the entire update process from the start with Magisk 18.1. Then when I rebooted from twrp (tried with and without cache cleared) it kept rebooting to twrp recovery. The install of magisk 20.1 zip via twrp initially also seemed to work. Then I re-installed twrp (3.3.1-1) successfully via adb fastboot. I still have all my original apps and settings). So what did I do so far… I started with a dirty flash of latest OTA (OnePluse3Oxygen_16_OTA_086_all_1911042118) which was successful.
I hope someone can advise me which version of Magisk and Magisk manager to install on my OP3 as I can not get it to work anymore after an update after trying multiple Magisk versions.