Last OOS was Android 11.
Quick search finds this:
https://service.oneplus.com/in/search/search-detail?id=2097031&articleIndex=1
Sometimes when updating phone the updates don’t always ping through immediately one after the other so you are in the land of educated guesswork as to what you had.
The fastboot method of flashing as far as I can tell is to restore the device to previous android version, that is: not to downgrade or upgrade.
Method would be to download the appropriate OOS rom and extract it somewhere on pc, then open terminal in that folder and issue the commands outlined in the first section of Manual Flash of this XDA topic. The fastboot partitions section. Don’t flash persist as it says in the other post you linked to.
I know you already tried this method and maybe it just won’t work for you. I helped a friend restore his OP6 a while ago, see this post here. That was about a year ago. Recently he tried it again but the flashing script would just not work, it put the phone in qualcomm crashdump mode. The only solution in the end was to use msm flashing method…which needs windows and setting up of appropriate drivers etc. See this: