Saved another 2019 Oneplus 7 that got stuck on Android 12 with a security update of somewhere in 2022.
It is now running Android 14 with the latest security update, thanks to @murena /e/os.
Biggest hurdle was to downgrade to Android 11, as Oneplus has created an issue with Oxygen OS -Android 12, where the OEM unlock has disappeared. Moreover, their downgrade links were obsolete. What a mess.
Installing /e/os was a breeze once unlocked.