One difference between the 9th gen and the 8th gen is
hardware protection against some of these security exploits, which were mentioned a while ago.
As the others say, sell it, take the hit, and move on with Zen2. Alternatively you could start a crusade by calling your local Citizen's Advice for returning faulty goods under legislation. However this is very time consuming and in the end it might be difficult to prove that these new security flaws can easily threat general home usage.
To be honest I don't give a **** about these exploits for home usage. These threats are a lot less severe than those 0 day exploits of Windows 10. The 8700K is still a decent choice for gaming.
For future purchases, to counter buyer's remorse, you could initiate a return request as unwanted for every order you make on the 14th day after delivery, so that you always have about one month of cooling-off period. 99% of the time I end up not returning anything, even if I initiate the return request, but this procedure gives you maximum rights.