@JoeM,
Sorry to hear about your trouble. But I think we can get this all sorted out as long as you’re willing to answer a few questions and work with me (and the rest of Community!) in troubleshooting this issue.
First, I would like to make sure we’re on the same page on a few things with respect to terminology and process:
Brave Muon - this is the “old Brave” that you had initially, before being upgraded in December. It looks like this:
We refer to it as “Muon” as that is the name of the engine used to drive it. Kind of a “project name”. Brave Muon is now depreciated (that is, no longer officially supported or updated) for several reasons, many announced earlier in Spring of last year.
Brave Core - this is the “new Brave” that you’re likely using right now (if you’re still with us! 
) and it looks like this:
It uses the open source Chromium Project engine, a very popular and efficient browser engine. We refer to it as “Brave Core” and it is several times faster and more effective than the previous Brave (Muon).
(Let me know in youre if you’d like to learn more about this aspect of Brave’s transition and I’d be happy to discuss it with you at length.)
This was the auto-upgrade that you received in December, prompting your initial post here in Community (assuming I’m understanding you correctly, please correct me if I’m wrong).
We posted a help document here on Community (which you referenced earlier) that detailed the basic process in which the update was supposed to go through as well as troubleshooting steps for users who had encountered problems during this process. At the top of the article:
This is what should have happened and (at least initially, as I’m not sure what steps have been taken since then) in all likelihood did happen. Reading further down:
This will be our first step to find your data. You’ve added the Linux tag to your post so I’m assuming you’re still using Ubuntu. Navigate to the ./config/ directory on your machine. Here, there should be at least one of two Brave related directories here:
.config/brave - Contains files and data from Brave Muon (Old).
.config/BraveSoftware Contains files and data from Brave Core (New).
If you’re looking for your old bookmarks and browsing data from the Brave Muon installation you had before being auto-updated in December, open the ./config/brave folder and look for a file called Session-store-1:
This file holds the majority of your browsing data, bookmarks, and saved passwords/login credentials. In the .config/brave directory, type cat session-store-1 into the terminal and hit enter to view the contents.
When you view the contents of this file, it will look very mixed and unformatted, but you should be able to identify some of your bookmarks or browsing information within the block of text:
Now, hopefully we are on the same page here at this point.
Please confirm for me whether or not you see your bookmarks or other relevant browsing data in the contents of your session-store-1 file.
We can determine the next best steps based on your findings. Again, if I’m misunderstanding your situation in anyway please let me know and if anything is unclear or you have further questions about anything mentioned so far please don’t hesitate to ask.