I have been messing around with my 6T trying out Postmarket OS w/Posh and Ubuntu Touch after having /e/ OS on the phone and ended up bricking one slot on the phone as a result of how /e/ OS uses the A/B slots. Since I am a daily Linux user for a long time I do not have access to a Windows machine to use the factory restore tool available on XDA (I think it is called MsMdownloadTool). I tried a Linux tool that is supposed to work the same but it was just extra problems.
In the end I found how to flash the stock Oxygen OS via Fastboot and made a bash script to automate the process on Linux. My script uses all the .img files from the fastboot 9.0.17 oxygen release found here.. My script is based off the windows scripts included in the download. Note: The included version of TWRP did not work for me and I had to use the one below.
TWRP Fajita version from here.