From what i have understood, the OP had an end date on his STV in September and then he has now obtained the last 90 day extension resulting in a final end date in December. Then after that time is wondering what will happen. At this moment you have two options. #1, 60 day covid extensions until no longer available, #2, Leave the country and come back.
I think its safe to say there will be at least one more covid extension announced. I feel like its a waste to leave now, when you can probably stay until mid 2022 on covid extensions.