Hi everyone,
It is indeed a rabbit hole Microsoft has us all down these days :) However, I finally got STOCKS working, and I noticed something not mentioned in these threads, that might be of help to others having problems.
First: Let me confirm what ended up working for me, as it has for others in this thread: I installed the older Excel 16.65 and then ran the License Removal Tool. This was essential to get things to work, signing out and in again was not enough.
Second: Here's the thing that may be of interest to others having problems, as there may be two issues muddling things for some people! Because after running the tool and then launching Excel, I saw the dialog in the screenshot below for the first time:

I had never seen this until I ran the License Removal Tool, and here is the key to why the tool was needed. It all has to do with how I (and probably many people) bought Office.
I first bought a license for Office Home & Business 2021 Mac so I could "graduate" from Apple Numbers to Excel, because I assumed Excel would have more advanced stocks and financial tools. Turns out Numbers was actually very close to Excel in this regard, and heaps easier to use, but I digress :)
But, as it turned out, I needed to buy a separate Microsoft 365 subscription to even use the stock-lookups and other tools. I mean, come on Microsoft! Well, whatever — I should have done my homework I suppose :)
But I bet many people will have bought Office Home and Business, thinking you could, oh I don't know, use it for business-stuff like stocks, and then found out they needed an extra subscription, bought that, and then found out it still did not work because of the current bug. So if you're in this situation, download Excel 16.65, run the License Tool, and then choose Microsoft 365 (Recommended) in the dialog seen above.
Because, I am correct that the 365 subscription is still needed, in order to use the Stocks function, right? It's getting confusing and I dare not mess more with it, so if anyone can confirm this is needed, I would be grateful.
Anyways, I hope this helps someone, and may I just say what a wonderful and supportive community I found here! You people are amazingly helpful and friendly. Special shoutout to to Jim Gordon for his incredible level of support here! How you get any other work done, I'll never know, but you're a fantastic ressource to have and I think I can say a huge Thank You from everyone here!
Cheers,
Thomas