Thank you alecxs for your support. I ended up reporting the phone to the police after successfully installing a new firmware on it. It kept restarting and showing a bicolour (green/blue) padlock which I assumed was a security feature from the carrier. The phone will work for extended periods of time, however, it would randomly restart when using standard ...
you can download rom for SM-G960F from updato, samfirm, frija or samloader. try lowest bootloader first, as you already stated downgrade is impossible (and higher version usually comes with higher protection against rooting). you can check bootloader version from download mode screen
Sony Xperia Phones have a very close to Stock Android UI eg Sony Xperia 1 II
You can learn more about the latest update to Android 11 for the aforementioned phone in the following Youtube Video:
Sony Xperia 1 II Update Android 11 | 15 New Features.
Hope this helps & Merry Christmas !