Officially the transition period can only be extended once. If it's to be extended beyond December 31st 2020, then the UK is required to request the extension by July 2020. Given that Boris campaigned on a manifesto of NOT extending the transition period, it's vanishingly unlikely that the UK will make such a request by July.
If we get to November/December 2020, and a trade deal is not ready, then there's the possibility that the UK and/or the EU would wish to implement some emergency extension of the transition period. It's very unclear at the moment how this might happen - either side or both sides might prefer a no-deal exit.
Most likely outcome, in my opinion, is that there will be a lightweight trade deal with zero tariffs in place. Discussions about various matters will be ongoing into 2021 and beyond but the official (fudged) line from both sides will be that Brexit has been successfully completed.
I think your analysis is entirely correct.
Some people seem to think that a) Brexit will signal some sort of abject collapse of the UK economy*, b) the trade deals that will (or will not) be implemented post-Brexit will necessarily greatly disadvantage the UK, c) there's effectively no way in which the Withdrawal Agreement can be sorted in any meaningful way by the end of 2020, and d) Johnson's agenda is in effect to deliberately kybosh the Withdrawal Agreement so that we leave with a
de facto no-deal Brexit.
But there seems to be an ongoing blindness to the fact that low-friction low-tariff (or tariff-free) reciprocating trade deals are exactly as much in the interests of the UK's key trade partners as they are of the UK itself. In addition, because the UK has never been within the Euro, there have had to be informal trade deals in place between the UK and the Eurozone in any case. And also, even trade deals with important non-EU countries need not be ridiculously arduous to sort out, since a) there are already trade deals in place between those countries and the EU, and b) those existing trade deals already ringfence the UK's participation in them, owing to our separate currency.
As ever, we'll see what happens on 1st January 2021. What I do
know is that this issue, just like so many other Brexit-related issues, has served as shiny ammunition for anti-Brexit and/or anti-Conservative-government commentators to predict some form of catastrophe. Colo(u)r me surprised......
* I am in little doubt that the UK's economy will be a worse shape after Brexit, compared with what it would have been were the UK remaining within the EU. And I'm a strong ideological Remain supporter. But we are where we are. The near-certain reality is that there will be a downward blip in the economy to account for things like rebasing of certain financial instruments/assets, and some slight form of downward adjustment of ongoing GDP forecasts. But against that, there will without doubt be an upward blip caused by the final removal of uncertainty (the markets and the big invetors fear uncertainty/volatility far more than they fear even lowered forecasts). So, IMO, the UK economy will more than survive Brexit: it's just that it won't do quite as well as it would probably have done had we remained.