Just download the ZIP archive of Magisk - https://magisk.me/zip/ and install it as a normal archive in TWRP
Brilliant thanks mate. For anyone else wondering, the magisk 23.0 zip from the link above works perfectly. TWRP is not required. I was able to reboot into the pixel experience recovery and flash it via adb connected to the pc. Root and magisk hide work perfectly. Safety net passes.
I also tried updating to magisk canary, however this would hard lock the phone 2 minutes after boot no matter what I did. Best to stick with 23.0.