@seth wellington
Try formatting data them reboot system. That should get you booted up.
Never wipe system or vendor on these devices and always when flashing a rom flash TWRP installer zip or you won't have twrp because of where recovery is located which is same place as boot.img and it has to be installed when flashing a rom or update to a rom or it won't be present.
How I flash OOS on my once T-Mobile now converted international/global OnePlus 6t.
Boot TWRP
Flash OOS global 10.3.6
Flash TWRP installer
Flash finalize
Format data
Reboot system
How I install a rom on my converted OnePlus 6t. This installation process varies depending on whether or not whatever rom I'm installing comes with gapps and or TWRP and of course you don't want to install global OOS on another model device. Since I converted my T-Mobile OnePlus 6t using guide and msm tool I now use global or international OnePlus 6t roms, firmware... Example Pixel Experience rom installation:
Boot TWRP
Flash OOS 10.3.6 global from repo xda thread
Flash Pixel Experience rom
Flash TWRP installer zip
Flash finalize zip
Format data
Reboot system.
Start to set up device then at some point enable developer options and usb debugging and doable any lock screen and boot twrp and flash magisk for root and reboot system. That is how I install a rom that has gapps but not twrp included in rom zip. Finalize can he found with a quick search on xda. It copies firmware to other slot on your AB partition device so that you don't have to. Saves 5-6 minutes and works great.
Edit
What you could have done is just flash your updated OOS , flash TWRP installer zip, reboot recovery, flash magisk, reboot system. That's what I would have done updating OOS ..
Hope this helps.
Sent from my OnePlus6T using XDA Labs