1 - forget the instructions given by the other kid - all wrong (like a lot, if not most, of the info given in theg5s plus threads.
2 - use twrp r15 - found in the forums, I think 16 is listed but I know 15 works. You must boot to twrp once after installing twrp or it'll get replaced with stock.
3 - you must format data to decrypt. Wiping only removes data - not the encryption
4 - I've never used magisk, but pretty sure it removed the encrypting flags when installed. I use su and no problems with stock - I've done it and know it works.
5 - download stock firmware, to fix booting to recovery you'll need stock recovery, and it's handy to have just in case.
What to do - install above twrp recovery - format data partition - install su - reboot. Done. If it is rebooting to recovery (which it shouldn't as long as you do boot to recovery after installing it) - flash stock recovery, reboot normally, then do the twrp install routine again.
Report back with details. No details - no help.
THX for that, got stuck in recovery process (loop).
I followed your instructions, but need of stock recovery.
My Path :
Backup DATA (w/o /media/)
Manually Backup Media with TWRP to Ext.SD
FORMAT /Data (use the DATA Button in TWRP), in my case it showed Errors, i ignored it and did it once again - this time it worked.
flashed Magisk
ended up in Recovery Loop - flashed stock recov. via FASTBOOT, reboot
a rainbow lightning circle turned up, said " DELETING", device rebooted by itself.
Now the Phone boots to Android, shut it down, restart with PWR BUTOON + VOL DOWN pressed to access bootloader and flash twrp again with FASTBOOT ( fastboot flash recovery nameoftheimage.img ENTER).
Now u can restore /DATA TWRP Backup and then the manually part - in my case that worked.
THX!
//EDIT
After first boot, i noticed missing write permissions for most Apps - so i decided to Reset SELinux via TWRP, worked !